Merge pull request kubernetes-sigs#10443 from Nordix/lentzi90/crs-docs-update

📖 Update docs for ClusterResourceSets

Author: k8s-ci-robot

docs/book/src/tasks/experimental-features/cluster-resource-set.md

@@ -8,9 +8,44 @@ The `ClusterResourceSet` feature is introduced to provide a way to automatically
 
 The `ClusterResourceSet` feature is enabled by default, but can be disabled by setting the `EXP_CLUSTER_RESOURCE_SET` environment variable to `false`.
 
-More details on `ClusterResourceSet` and an example to test it can be found at:
+More details on `ClusterResourceSet` can be found at:
 [ClusterResourceSet CAEP](https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20200220-cluster-resource-set.md)
 
+## Example
+
+Suppose you want to automatically install the relevant external cloud provider on all workload clusters.
+This can be accomplished by labeling the clusters with the specific cloud (e.g. AWS, GCP or OpenStack) and then creating a `ClusterResourceSet` for each.
+For example, you could have the following for OpenStack:
+
+```yaml
+apiVersion: addons.cluster.x-k8s.io/v1beta1
+kind: ClusterResourceSet
+metadata:
+  name: cloud-provider-openstack
+  namespace: default
+spec:
+  strategy: Reconcile
+  clusterSelector:
+    matchLabels:
+      cloud: openstack
+  resources:
+    - name: cloud-provider-openstack
+      kind: ConfigMap
+    - name: cloud-config
+      kind: Secret
+```
+
+This `ClusterResourceSet` would apply the content of the `Secret` `cloud-config` and of the `ConfigMap` `cloud-provider-openstack` in all workload clusters with the label `cloud=openstack`.
+Suppose you have the file `cloud.conf` that should be included in the `Secret` and `cloud-provider-openstack.yaml` that should be in the `ConfigMap`.
+The `Secret` and `ConfigMap` can then be created in the following way:
+
+```bash
+kubectl create secret generic cloud-config --from-file=cloud.conf --type=addons.cluster.x-k8s.io/resource-set
+kubectl create configmap cloud-provider-openstack --from-file=cloud-provider-openstack.yaml
+```
+
+Note that it is required that the `Secret` has the type `addons.cluster.x-k8s.io/resource-set` for it to be picked up.
+
 ## Update from `ApplyOnce` to `Reconcile`
 
 The `strategy` field is immutable so existing CRS can't be updated directly. However, CAPI won't delete the managed resources in the target cluster when the CRS is deleted.