Add helm-controller docs

docs/components/helm/controller.md

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+# Helm Controller
+
+The Helm Controller is a Kubernetes operator, allowing one to declaratively manage Helm chart
+releases with Kubernetes manifests.
+
+![](../../_files/helm-controller.png)
+
+The desired state of a Helm release is described through a Kubernetes Custom Resource named `HelmRelease`.
+Based on the creation, mutation or removal of a HelmRelease resource in the cluster,
+Helm actions are performed by the controller.
+
+Features:
+
+- Watches for `HelmRelease` objects and generates `HelmChart` objects
+- Fetches artifacts produced by [source-controller](../source/controller.md) from `HelmChart` objects
+- Watches `HelmChart` objects for revision changes (semver ranges)
+- Performs Helm v3 actions including Helm tests as configured in the `HelmRelease` objects
+- Runs Helm install/upgrade in a specific order, taking into account the depends-on relationship
+- Prunes Helm releases removed from cluster (garbage collection) 
+- Reports Helm releases status (alerting provided by [notification-controller](../notification/controller.md))
+
+Links:
+
+- Source code [fluxcd/helm-controller](https://github.com/fluxcd/helm-controller)
+- Specification [docs](https://github.com/fluxcd/helm-controller/tree/master/docs/spec)