Clearfix change from v3 should be listed under breaking changes

Created by: geoffbeaumont

Bootstrap clearfix has only an :after, the :before pseudo element has been dropped. Not sure why as the old version was far more useful (contains top margins within the element), and I can't think of an instance where the new version would be better.

More to the point, however - this will break a lot of existing layouts, so needs to be listed under breaking changes (or the :before restored, which would be my preference).