meta: add "joining a wg" section to WORKING_GROUPS.md

In the WORKING_GROUPS.md documentation, it is described how to start a
wg, but not how to join an existing wg. This commit addresses that
issue.

Fixes: https://github.com/nodejs/node/issues/5448
PR-URL: https://github.com/nodejs/node/pull/5488
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: James M Snell <jasnell@gmail.com>

WORKING_GROUPS.md

@@ -300,6 +300,11 @@ It's responsibilities are:
 * Working with the Build Working Group to improve continuous integration.
 * Improving tooling for testing.
 
+## Joining a WG
+
+To find out how to join a working group, consult the GOVERNANCE.md in
+the working group's repository, or simply open an issue there.
+
 ## Starting a WG
 
 A Working Group is established by first defining a charter  that can be