Merge pull request #12893 from chrischdi/pr-contract-cp-label

📖 Update control-plane contract to mention the label cluster.x-k8s.io/control-plane

Author: k8s-ci-robot

docs/book/src/developer/providers/contracts/control-plane.md

@@ -420,7 +420,9 @@ policies when a Cluster derived from a ClusterClass is managed by the Topology c
 ### ControlPlane: machines
 
 In case you are developing a control plane provider which uses a Cluster API Machine object to represent each
-control plane instance, following fields MUST be implemented in
+control plane instance, the providers MUST set the `cluster.x-k8s.io/control-plane` label with an empty value on the created Machines.
+
+Additionally following the fields MUST be implemented in
 the ControlPlane `spec`.
 
 ```go