Bug #1328716: Clean up CONTRIBUTING a bit.

Per Bug #1328716, here's a patch that updates CONTRIBUTING to mesh better with README.  It also fills in some gaps, particularly with tools, and makes the workflow more clear.  Given that working on this patch is my first exposure to the workflow I have no illusions as to perfect accuracy.  So feedback would be much appreciated.
You mean the "so plan on using git for your contributions" part?  Yeah, it sounds a little pushy. :)  I added it to make the sentences flow a little better.  You think I should drop it?