This would contain a SHORT description of what Greasemonkey is (and maybe UserJS, GreaseKit etc.) and how to install scripts (so new users won't ask this in forums).

Followed of course by a nice friendly welcome from the site admins :) and then whatever else. Maybe a short FAQ for new people as well, or links to reccommended reading. Just general helpfullness.

P.S. I think this could be seen as a duplicate of some other ideas mentioned before, but I have read through them all and they are either poorly worded and difficult to understand, or less specific (there's an idea suggesting having links to an FAQ page - my idea just suggests a helpful frontpage so there's no need for FAQ)