docs: supplement plugin creation

docs/book/src/plugins/creating-plugins.md

@@ -12,6 +12,40 @@
 
 You can extend the Kubebuilder API to create your own plugins. If [extending the CLI][extending-cli], your plugin will be implemented in your project and registered to the CLI as has been done by the [SDK][sdk] project. See its [CLI code][sdk-cli-pkg] as an example.
 
+## When it is useful?
+
+- If you are looking to create plugins which support and work with another language.
+- If you would like to create helpers and integrations on top of the scaffolds done by the plugins provided by Kubebuiler.
+- If you would like to have customized layouts according to your needs.
+
+## How the plugins can be used?
+
+Kubebuilder provides a set of plugins to scaffold the projects, to help you extend and re-use its implementation to provide additional features.
+For further information see [Available Plugins][available-plugins].
+
+Therefore, if you have a need you might want to propose a solution by adding a new plugin
+which would be shipped with Kubebuilder by default.
+
+However, you might also want to have your own tool to address your specific scenarios and by taking advantage of what is provided by Kubebuilder as a library.
+That way, you can focus on addressing your needs and keep your solutions easier to maintain.
+
+Note that by using Kubebuilder as a library, you can import its plugins and then create your own plugins that do customizations on top.
+For instance, `Operator-SDK` does with the plugins [manifest][operator-sdk-manifest] and [scorecard][operator-sdk-scorecard] to add its features.
+Also see [here][operator-sdk-plugin-ref].
+
+Another option implemented with the [Extensible CLI and Scaffolding Plugins - Phase 2][plugins-phase2-design-doc] is
+to extend Kibebuilder as a LIB to create only a specific plugin that can be called and used with
+Kubebuilder as well.
+
+<aside class="note">
+<H1> Plugins proposal docs</H1>
+
+You can check the proposal documentation for better understanding its motivations. See the [Extensible CLI and Scaffolding Plugins: phase 1][plugins-phase1-design-doc],
+the [Extensible CLI and Scaffolding Plugins: phase 1.5][plugins-phase1-design-doc-1.5] and the [Extensible CLI and Scaffolding Plugins - Phase 2][plugins-phase2-design-doc]
+design docs. Also, you can check the [Plugins section][plugins-section].
+
+</aside>
+
 ## Language-based Plugins
 
 Kubebuilder offers the Golang-based operator plugins, which will help its CLI tool users create projects following the [Operator Pattern][operator-pattern].
@@ -68,5 +102,97 @@ kubebuilder edit --plugins="grafana.kubebuilder.io/v1-alpha"
 
 In this way, by [Extending the Kubebuilder CLI][extending-cli], you can also create custom plugins such this one.
 
-[grafana]: https://github.com/kubernetes-sigs/kubebuilder/tree/v3.7.0/pkg/plugins/optional/grafana/v1alpha
+Feel free to check the implementation under:
+
+- deploy-image: <https://github.com/kubernetes-sigs/kubebuilder/tree/v3.7.0/pkg/plugins/golang/deploy-image/v1alpha1>
+- grafana: <https://github.com/kubernetes-sigs/kubebuilder/tree/v3.7.0/pkg/plugins/optional/grafana/v1alpha>
+
+## Plugin Scaffolding
+
+Your plugin may add code on top of what is scaffolded by default with Kubebuilder sub-commands(`init`, `create`, ...).
+This is common as you may expect your plugin to:
+
+- Create API
+- Update controller manager logic
+- Generate corresponding manifests
+
+### Boilerplates
+
+The Kubebuilder internal plugins use boilerplates to generate the files of code.
+
+For instance, the go/v3 scaffolds the `main.go` file by defining an object that [implements the machinery interface][kubebuilder-machinery].
+In the [implementation][go-v3-settemplatedefault] of `Template.SetTemplateDefaults`, the [raw template][go-v3-main-template] is set to the body.
+Such object that implements the machinery interface will later pass to the [execution of scaffold][go-v3-scaffold-execute].
+
+Similar, you may also design your code of plugin implementation by such reference.
+You can also view the other parts of the code file given by the links above.
+
+If your plugin is expected to modify part of the existing files with its scaffold, you may use functions provided by [sigs.k8s.io/kubebuilder/v3/pkg/plugin/util][kb-util].
+See [example of deploy-image][example-of-deploy-image-2].
+In brief, the util package helps you customize your scaffold in a lower level.
+
+### Use Kubebuilder Machinery Lib
+
+Notice that Kubebuilder also provides [machinery pkg][kubebuilder-machinery-pkg] where you can:
+
+- Define file I/O behavior.
+- Add markers to the scaffolded file.
+- Define the template for scaffolding.
+
+#### Overwrite A File
+
+You might want for example to overwrite a scaffold done by using the option:
+
+```go
+	f.IfExistsAction = machinery.OverwriteFile
+```
+
+Let's imagine that you would like to have a helper plugin that would be called in a chain with `go/v4-alpha` to add customizations on top.
+Therefore after we generate the code calling the subcommand to `init` from `go/v4-alpha` we would like to overwrite the Makefile to change this scaffold via our plugin.
+In this way, we would implement the Bollerplate for our Makefile and then use this option to ensure that it would be overwritten.
+
+See [example of deploy-image][example-of-deploy-image-1].
+
+### A Combination of Multiple Plugins
+
+Since your plugin may work frequently with other plugins, the executing command for scaffolding may become cumbersome, e.g:
+
+```shell
+kubebuilder create api --plugins=go/v3,kustomize/v1,yourplugin/v1
+```
+
+You can probably define a method to your scaffolder that calls the plugin scaffolding method in order.
+See [example of deploy-image][example-of-deploy-image-3].
+
+#### Define Plugin Bundles
+
+Alternatively, you can create a plugin bundle to include the target plugins. For instance:
+
+```go
+  mylanguagev1Bundle, _ := plugin.NewBundle(language.DefaultNameQualifier, plugin.Version{Number: 1},
+        kustomizecommonv1.Plugin{}, // extend the common base from Kuebebuilder
+        mylanguagev1.Plugin{}, // your plugin language which will do the scaffolds for the specific language on top of the common base
+    )
+```
+
+[controller-runtime]: https://github.com/kubernetes-sigs/controller-runtime
 [deploy-image]: https://github.com/kubernetes-sigs/kubebuilder/tree/v3.7.0/pkg/plugins/golang/deploy-image/v1alpha1
+[grafana]: https://github.com/kubernetes-sigs/kubebuilder/tree/v3.7.0/pkg/plugins/optional/grafana/v1alpha
+[extending-cli]: ./extending-cli.md
+[kb-util]: https://pkg.go.dev/sigs.k8s.io/kubebuilder/v3/pkg/plugin/util
+[example-of-deploy-image-1]: https://github.com/kubernetes-sigs/kubebuilder/blob/df1ed6ccf19df40bd929157a91eaae6a9215bfc6/pkg/plugins/golang/deploy-image/v1alpha1/scaffolds/internal/templates/api/types.go#L58
+[example-of-deploy-image-2]: https://github.com/kubernetes-sigs/kubebuilder/blob/master/pkg/plugins/golang/deploy-image/v1alpha1/scaffolds/api.go#L170-L266
+[example-of-deploy-image-3]: https://github.com/kubernetes-sigs/kubebuilder/blob/master/pkg/plugins/golang/deploy-image/v1alpha1/scaffolds/api.go#L77-L98
+[available-plugins]: https://github.com/kubernetes-sigs/kubebuilder/pull/available-plugins.md
+[operator-sdk-manifest]: https://github.com/operator-framework/operator-sdk/tree/v1.23.0/internal/plugins/manifests/v2
+[operator-sdk-scorecard]: https://github.com/operator-framework/operator-sdk/tree/v1.23.0/internal/plugins/scorecard/v2
+[operator-sdk-plugin-ref]: https://github.com/operator-framework/operator-sdk/blob/v1.23.0/internal/cmd/operator-sdk/cli/cli.go#L78-L160
+[plugins-section]: ???
+[plugins-phase1-design-doc]: https://github.com/kubernetes-sigs/kubebuilder/blob/v3.7.0/designs/extensible-cli-and-scaffolding-plugins-phase-1.md#extensible-cli-and-scaffolding-plugins
+[plugins-phase1-design-doc-1.5]: https://github.com/kubernetes-sigs/kubebuilder/blob/v3.7.0/designs/extensible-cli-and-scaffolding-plugins-phase-1-5.md#extensible-cli-and-scaffolding-plugins---phase-15
+[plugins-phase2-design-doc]: https://github.com/kubernetes-sigs/kubebuilder/blob/v3.7.0/designs/extensible-cli-and-scaffolding-plugins-phase-2.md#extensible-cli-and-scaffolding-plugins---phase-2
+[go-v3-main-template]: https://github.com/kubernetes-sigs/kubebuilder/blob/3bfc84ec8767fa760d1771ce7a0cb05a9a8f6286/pkg/plugins/golang/v3/scaffolds/internal/templates/main.go#L183
+[kubebuilder-machinery]: https://github.com/kubernetes-sigs/kubebuilder/blob/3bfc84ec8767fa760d1771ce7a0cb05a9a8f6286/pkg/plugins/golang/v3/scaffolds/internal/templates/main.go#L28
+[go-v3-settemplatedefault]: https://github.com/kubernetes-sigs/kubebuilder/blob/3bfc84ec8767fa760d1771ce7a0cb05a9a8f6286/pkg/plugins/golang/v3/scaffolds/internal/templates/main.go#L40
+[go-v3-scaffold-execute]: https://github.com/kubernetes-sigs/kubebuilder/blob/3bfc84ec8767fa760d1771ce7a0cb05a9a8f6286/pkg/plugins/golang/v3/scaffolds/init.go#L120
+[kubebuilder-machinery-pkg]: https://pkg.go.dev/sigs.k8s.io/kubebuilder/v3/pkg/machinery#section-documentation