document workaround for #676 (#677)

Author: Feder1co5oave

docs/bgp.md

@@ -62,6 +62,8 @@ for Route Reflector client mode.
 
 Only nodes with the same ClusterID in client and server mode will peer together.
 
+When joining new nodes to the cluster, remember to annotate them with `kube-router.io/rr.client=42`, and then restart kube-router on the new nodes and the route reflector server nodes to let them successfully read the annotations and peer with each other.
+
 ## Peering Outside The Cluster
 ### Global External BGP Peers