[scikit-learn] Development workflow proposal: merge master instead of rebasing

Hi all. I think we should change our development practices for resolving merge-conflicts from rebasing to merging. The "squash and merge" button of github gets rid of any merge commits and results in a clean history in any case. The benefit of merging instead of rebasing is that github is able to track comments much better if you don't force-push. In particular the links in notification emails might work better when not doing force-pushes. I'm not entirely sure how the mechanism works, but I think it's worth giving it a go. When merging master it's also harder to screw up an PR entirely (I think) which would make it easier to people new to git. Wdyt? Andy