Update CONTRIBUTING to reflect ReviewBoard.
Add a note about falling back to github.
Update to skip manual steps.
Add a document describing ReviewBoard and how we use it.
Update the text.
Move rbt auth info down to the rbt section.
Merge pull request #1173 from ericsnowcurrently/reviewboard-contributing

Update CONTRIBUTING to reflect ReviewBoard.



(Review request: http://reviews.vapour.ws/r/484/)