Release process docs

Hi,

I've added a brain dump of the things I do when I release rails. You can find it here:

  https://github.com/rails/rails/blob/master/RELEASING_RAILS.rdoc

Please feel free to update it with formatting, better spelling, clarity, etc.

There is no tl;dr because you should read the whole thing. Once we all get used to releasing, we should probably put together a checklist of some sort. This document is targeted towards people who have never released rails (or need a refresher course).

I've left a few FIXME's in the document of things I can't remember right now and places where we should improve.