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.
I would change 'as normal' to 'in the usual way'. Also, I would remove the "; the same commits shouldn't normally need to go through the issue resolution process for each active topic branch" clause.