fix(Documentation/cluster-discovery): link to optimal cluster size

Documentation/cluster-discovery.md

@@ -6,7 +6,9 @@ Starting an etcd cluster can be painful since each node needs to know of another
 
 Peer discovery for etcd is processed by `-discovery`, `-peers` and lastly log data in `-data-dir`. For more information see the [discovery design][discovery-design].
 
-Please note - at least 3 nodes are required for cluster availability.
+Please note - at least 3 nodes are required for [cluster availability][optimal-cluster-size].
+
+[optimal-cluster-size]: https://github.com/coreos/etcd/blob/master/Documentation/optimal-cluster-size.md
 
 ## Using discovery.etcd.io