Document topic branches

Since we made the graph branch, and since I just moved the branch pointer unilaterally, I figured I ought to legitimize such things. This commit updates the contributing document to describe how I think topic branches like this ought to be handled.
Require review of all (and only) actual commits

If you want to fast-forward a branch up to master you can, but if there is actual merging involved you need to pull request it for review.
Making Jerome's changes

Hopefully slightly clearer to read now. @jeromekelleher?
Merge pull request #199 from adamnovak/styleGuide

Document topic branches