[release-1.2] 📖 Add cluster-resource-set-controller to docs

docs/book/src/SUMMARY.md

@@ -61,6 +61,7 @@
         - [Control Plane](./developer/architecture/controllers/control-plane.md)
         - [MachinePool](./developer/architecture/controllers/machine-pool.md)
         - [ClusterTopology](./developer/architecture/controllers/cluster-topology.md)
+        - [ClusterResourceSet](./developer/architecture/controllers/cluster-resource-set.md)
         - [Metadata propagation](./developer/architecture/controllers/metadata-propagation.md)
         - [Multi-tenancy](./developer/architecture/controllers/multi-tenancy.md)
         - [Support multiple instances](./developer/architecture/controllers/support-multiple-instances.md)

docs/book/src/developer/architecture/controllers.md

@@ -16,6 +16,8 @@ Documentation for the CAPI controllers can be found at:
   - [MachinePool](./controllers/machine-pool.md)
  - ClusterClass
   - [Cluster Topology](./controllers/cluster-topology.md)
+- AddOns
+   - [ClusterResourceSet](./controllers/cluster-resource-set.md)
 
 
 <!-- links -->

docs/book/src/developer/architecture/controllers/cluster-resource-set.md

@@ -0,0 +1,12 @@
+# `ClusterResourceSet` Controller
+
+The `ClusterResourceSet` provides a mechanism for applying resources - e.g. pods, deployments, daemonsets, secrets, configMaps - to a cluster once it is created.
+
+![ClusterResourceSet Controller Activity Diagram](../../../images/cluster-resource-set-controller.png)
+
+Its main responsibility is to automatically apply a set of resources to newly-created and existing Clusters. Resources will be applied only once.
+
+
+### Additional information
+
+* See ClusterResourceSet [proposal](https://github.com/kubernetes-sigs/cluster-api/blob/main/docs/proposals/20200220-cluster-resource-set.md)

docs/book/src/images/cluster-resource-set-controller.plantuml

@@ -0,0 +1,28 @@
+@startuml
+!pragma useVerticalIf on
+start
+:ClusterResourceSet Controller;
+repeat
+  :ClusterResourceSet controller enqueues a Reconcile call;
+  :Fetch the ClusterResourceSet instance in namespace;
+  repeat
+    if (Cluster matches ClusterResourceSet's selector) then (yes)
+      if (Get ClusterResourceSetBinding for the cluster) then (exists)
+      else (doesn't exist)
+         :Create ClusterResourceSetBinding object for the cluster;
+      endif
+      if (ResourceSetBinding for a given ClusterResourceSet exists) then (yes)
+      else (no)
+        :Create ResourceSetBinding;
+      endif
+    :Get Resoures in ClusterResourceSet;
+    if (Check applied condition of resource in ResourceSetBinding) then (false)
+     :Apply resource to cluster;
+     :Set applied condition to true in ResourceSetBinding;
+    endif
+    :Mark ResourcesAppliedCondition to true;
+  endif
+  repeat while (More clusters?)
+repeat while (More updates to ClusterResourceSet?)
+stop
+@enduml