Improve login information for existing users on make/chat

[Clorith]

The current wording on make/chat may not always be accurate, it states the following:

You’ve already been invited to wordpress.slack.com. Go there to log in. Be sure to log in using $username@chat.wordpress.org as your email address.

As you can change your email at will after having been initially invited, we should add a note that this may no longer be your actual login name, and that typing in your email address can send you an email from Slack with all the teams that address has access to.