gut README.md and insert links to wiki documentation

Almost all information has been removed from the file and merged into the wiki pages that are now linked to in the file. Approval to do this from lefnire is in https://github.com/HabitRPG/habitrpg/pull/4024#issuecomment-55451042

Feedback welcome. I'll merge this in a couple of days if there's no objections.

@thepeopleseason : I'd like to do something similar for [VAGRANT.md](https://github.com/HabitRPG/habitrpg/blob/develop/VAGRANT.md) if you don't object, so that we have all documentation in one place only, for easier updating. Would that be okay?