Added contributing policy

Fixes #14.
> 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.
No, they stay because you commented on the diff, not on the commits themselves.

I'm really a fan of clean commit histories. Also, the policy clearly states `If you know how to use "git rebase"`, so nobody will be forced to do anything he/she isn't capable of doing.

But if you both agree, I'll remove that section.