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>