Add some very rudimentary testing tips

Adds basic pointers to an example in the reference section.

docs/book/src/SUMMARY.md

@@ -68,6 +68,7 @@
 
   - [controller-gen CLI](./reference/controller-gen.md)
   - [Artifacts](./reference/artifacts.md)
+  - [Writing controller tests](./reference/writing-tests.md)
 
 ---
 

docs/book/src/reference/writing-tests.md

@@ -0,0 +1,16 @@
+# Writing controller tests
+
+Testing Kubernetes controller is a big subject, and the boilerplate testing
+files generated for you by kubebuilder are fairly minimal. Until more
+documentation has been written, your best bet to get started is to look at some
+existing examples, such as:
+
+* Azure Databricks Operator: see their fully fleshed-out
+  [`suite_test.go`](https://github.com/microsoft/azure-databricks-operator/blob/0f722a710fea06b86ecdccd9455336ca712bf775/controllers/suite_test.go)
+  as well as any `*_test.go` file in that directory [like this
+  one](https://github.com/microsoft/azure-databricks-operator/blob/0f722a710fea06b86ecdccd9455336ca712bf775/controllers/secretscope_controller_test.go).
+
+The basic approach is that, in your generated `suite_test.go` file, you will
+create a local Kubernetes API server, instantiate and run your controllers, and
+then write additional `*_test.go` files to test it using
+[Ginko](http://onsi.github.io/ginkgo).