Add usage for "join" without an argument

When we were using `wemux` today, we wanted to know what the current server was.  However, we didn't see an option for this in `wemux help`.  I looked into it (thinking it would be a good feature addition), but then found out that `wemux join` without an argument did what we wanted.  So I documented it.  :smile:

Thanks for maintaining `wemux`!

Ben