Introducing a CONTRIBUTING guide

Okay, I understand your point - but I disagree for the following reasons:

* we can't assume users _do_ know about organizing branch history with rebase. For that reason, we need to make an effort at targeting novices; experts will gloss the content and move on.

* for those users that are not experts, at least the link is a starting point for them to learn more about rebasing. Sure it's not perfect, or the _definitive guide_, but it's better than nothing.

I'd rather have documentation that is overly-explicit and assumption-free, than one that values removing tautology at the expense of user understanding...