Add note about namespace methods

Author: nuclearace

Usage Docs/12to13.md

@@ -47,6 +47,10 @@ to the manager.
 
 - `SocketIOClient`s no longer take a configuration, they are shared from the manager.
 
+- The `joinNamespace()` and `leaveNamespace()` methods on `SocketIOClient` no longer take any arguments, and in most cases
+no longer need to be called. Namespace joining/leaving can be managed by calling `connect()`/`disconnect()` on the socket
+associated with that namespace.
+
 ----------
 
 # What things should I know?