Add docs on how to use Kustomize to generate Helm values

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

docs/guides/helmreleases.md

@@ -257,6 +257,83 @@ The definition of the listed keys is as follows:
     You can read more about the available formats and limitations in
     the [Helm documentation](https://helm.sh/docs/intro/using_helm/#the-format-and-limitations-of---set).
 
+## Refer to values in `ConfigMaps` generated with Kustomize
+
+It is possible to use Kustomize [ConfigMap generator](https://kubectl.docs.kubernetes.io/references/kustomize/configmapgenerator/)
+to trigger a Helm release upgrade every time the encoded values change.
+
+First create a `kustomizeconfig.yaml` for Kustomize to be able to patch
+`ConfigMaps` referenced in `HelmRelease` manifests:
+
+```yaml
+nameReference:
+- kind: ConfigMap
+  version: v1
+  fieldSpecs:
+  - path: spec/valuesFrom/name
+    kind: HelmRelease
+```
+
+Create a `HelmRelease` definition that references a `ConfigMap`:
+
+```yaml
+apiVersion: helm.toolkit.fluxcd.io/v2beta1
+kind: HelmRelease
+metadata:
+  name: podinfo
+  namespace: podinfo
+spec:
+  interval: 5m
+  releaseName: podinfo
+  chart:
+    spec:
+      chart: podinfo
+      sourceRef:
+        kind: HelmRepository
+        name: podinfo
+  valuesFrom:
+    - kind: ConfigMap
+      name: podinfo-values
+```
+
+Create a `kustomization.yaml` that generates the `ConfigMap` using our kustomize config:
+
+```yaml
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+namespace: podinfo
+resources:
+  - namespace.yaml
+  - repository.yaml
+  - release.yaml
+configMapGenerator:
+  - name: podinfo-values
+    files:
+      - values.yaml=my-values.yaml
+configurations:
+  - kustomizeconfig.yaml
+```
+
+When [kustomize-controller](../components/kustomize/controller.md) reconciles the above manifests, it will generate
+a unique name of the `ConfigMap` every time `my-values.yaml` content is updated in Git:
+
+```yaml
+apiVersion: helm.toolkit.fluxcd.io/v2beta1
+kind: HelmRelease
+metadata:
+  name: podinfo
+  namespace: podinfo
+spec:
+  valuesFrom:
+  - kind: ConfigMap
+    name: podinfo-values-2mh2t8m94h
+```
+
+!!! hint "Note"
+    Stale `ConfigMaps`, previously generated by Kustomize, will be
+    removed from the cluster by kustomize-controller if
+    [pruning](../components/kustomize/kustomization/#garbage-collection) is enabled.
+
 ## Refer to values inside the chart
 
 It is possible to replace the `values.yaml` with a different file present inside the Helm chart.