Added contributing policy

Fixes #14.
> It rewrites history

But only pull request history. It has not become project history yet, therefore it is not important. I agree that you should never ever rebase the master branch. But nobody should check out a pull request branch and start to use it productively.

Many commit messages that made sense in the context of a pull request don't make sense in the context of the full repository history anymore.

It also prevents that a commit for a feature contains great documentation on the reasons for this commit. While that reasoning might be present in a pull request discussion, that won't be obvious to a user or dev later on. If it's in the git history, it can be looked up.

> and it interferes with people collaborating on the same PR.

Usually only one person should contribute per pull request (unless the people are in the same room and know they don't cause conflicts).

> One feature can be traced back to one commit anyway: the merge commit

A merge commit doesn't show the entire diff.