docs: Document the possibility of rendering Helm charts with Kustomize (

#9841)

* Update kustomize.md

Resolves  #7835.

Signed-off-by: Didrik Finnøy <djfinnoy@protonmail.com>

* Removed unnecessary command flag from example. Minor text edits.

Signed-off-by: Didrik Finnøy <djfinnoy@protonmail.com>

* spelling

Signed-off-by: Didrik Finnøy <djfinnoy@protonmail.com>

docs/user-guide/kustomize.md

@@ -85,3 +85,22 @@ argocd app set <appName> --kustomize-version v3.5.4
 ## Build Environment
 
 Kustomize does not support parameters and therefore cannot support the standard [build environment](build-environment.md).
+
+## Kustomizing Helm charts
+
+It's possible to [render Helm charts with Kustomize](https://github.com/kubernetes-sigs/kustomize/blob/master/examples/chart.md).
+Doing so requires that you pass the `--enable-helm` flag to the `kustomize build` command.
+This flag is not part of the Kustomize options within Argo CD.
+If you would like to render Helm charts through Kustomize in an Argo CD application, you have two options:
+You can either create a [custom plugin](https://argo-cd.readthedocs.io/en/stable/user-guide/config-management-plugins/), or modify the `argocd-cm` ConfigMap to include the `--enable-helm` flag globally for all Kustomize applications:
+
+```
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: argocd-cm
+  namespace: argocd
+data:
+  kustomize.buildOptions: --enable-helm
+```
+