Write add-on help

[Robert Bergman]

The add-on needs some minimal documentation, at least for MPAC. Particularly, we need instructions on what to do when a room's email address is compromised. Currently the solution is simply to reinstall (or uninstall) the add-on. A /command (or simply output when the default /mailroom command is run) to get this advice in the chatroom would be useful as well.

[Robert Bergman]

Already live at mailroom.hipch.at.