Added contributing policy

Fixes #14.
Damn it, looks like I'm too late. I really really *really* like the use of git-rebase to produce clean and easy-to-review histories; I use it for that at work every day. The commit history should, IMHO, tell a *useful* story about how the code was developed; this is not always the same thing as a *literally correct* history. The comments are not lost by GitHub: they show up as "commented on an outdated diff" links. But since most of the uses of this feature are to address minor issues, you don't usually need to read them anyway.

On [http://github.com/Growstuff/growstuff](Growstuff), "reorder your commits if you know how" is stated policy, but I think I'm the only one who actually does. So I don't see any problem with having an "if you know how" commit-rewriting policy.