Added contributing policy

Fixes #14.
### Well Known Projects Maintaining Clean History
Django and Celery are great examples of this but for every project doing this there are major projects not doing it. Rebasing and squashing for this purpose has, to my mind anyway, always appeared to be a very philosophical project specific decision. Personally I feel it raises the barrier of entry to a project, which I'm keen to avoid. Taking Django as an example, squashing is typically done by the merging developer, not the person raising the PR, to avoid the raiser having to know how to do it. This puts the burden of knowing how to squash and rebase on the core dev team. Now, I'm more than capable of doing some fairly advanced things with git's history manipulation tools but I don't think this should be a requirement of any of the other developers, nor do I want to have to get involved with PRs myself.

### It's an Advanced Feature
Rebasing is, at the end of the day, one of the most advanced features in Git. Adding such a powerful and dangerous tool as a suggested method for contribution (even an optional one) is likely to bite us in the long run.

### Maintaining History
Personally I dislike the use of rebase once code has been pushed, it's far too easy to impact other developers (whether you mean to or not) and rewriting history can have nasty knock on effects. Take this PR: if you rebase the history now where do all our comments go? IIRC they disappear because they're tied to the SHAs of your commits. I'd much rather have some messy commits which only make contextual sense in the history *and* the PRs on GitHub than clean commits in the history.

As @doismellburning has mentioned there are tools for viewing even messy commits with context on the CLI and every GUI tool I know of can do this.

### TL;DR
I love using rebase and having a clean history but have been bitten so many times by pushing rebased commits to collaborative projects that I cannot in any way condone adding this to the contributing docs.