Add excerpt CONTRIBUTING.md about rewriting branches for PRs #151
Description
An essential skill for contributing to patch-hub and, IMHO, in any project, is to be able to rewrite a branch using git rebase --interactive + git commit --amend. Yet, we can't expect newcomers who are eager to contribute to have this previous knowledge, nor should we ignore contributors who don't know about this.
I think the "Pull Requests Guidelines" section inside the CONTRIBUTING.md is a good place to formalize this "practice/pre-requisite" and maybe add this long-time reference.
We shouldn't just point to the reference (like, "pre-requisite: read this) and be done with it, though; we should at least elaborate about (1) what we are talking about and (2) why this is important for people that are not familiar with this, but also people who are, so they don't need to start reading the document to say "Oh, I know about this" or to make them understand the importance of it, if they don't.