doc the Kubernetes version feature of KubeFedCluster

Author: zqzten

docs/cluster-registration.md

@@ -34,12 +34,14 @@ Check the status of the joined clusters by using the following command.
 ```bash
 kubectl -n kube-federation-system get kubefedclusters
 
-NAME       READY   AGE
-cluster1   True    1m
-cluster2   True    1m
+NAME       AGE   READY   KUBERNETES-VERSION
+cluster1   1m    True    v1.21.2
+cluster2   1m    True    v1.22.0
 
 ```
 
+The Kubernetes version is checked periodically along with the cluster health check so that it would be automatically updated within the cluster health check period after a Kubernetes upgrade/downgrade of the cluster.
+
 # Joining kind clusters on MacOS
 
 A Kubernetes cluster deployed with [kind](https://sigs.k8s.io/kind) on Docker