docs(CONTRIBUTING.md): Added a not about type

It's important that we let people use the GitHub editing interface without being 100% strict about how to name the commit changes. Otherwise, it is basically a barrier to entry and highly discouraging for new people who may just be trying to fix a spelling error. Since it is possible for contributors to edit the commit message before merging it into master, for people who are new to the commit styling system, we should be lenient about minor infractions like forgetting to put docs: in front of a message. 

CF: https://github.com/angular-ui/bootstrap/pull/2635#issuecomment-57117579