Adding clusters to the list of valid resources printed in kubectl help

pkg/kubectl/cmd/cmd.go

@@ -158,7 +158,9 @@ __custom_func() {
 
 	// If you add a resource to this list, please also take a look at pkg/kubectl/kubectl.go
 	// and add a short forms entry in expandResourceShortcut() when appropriate.
+	// TODO: This should be populated using the discovery information from apiserver.
 	valid_resources = `Valid resource types include:
+   * clusters (valid only for federation apiservers)
    * componentstatuses (aka 'cs')
    * configmaps (aka 'cm')
    * daemonsets (aka 'ds')