Added contributing policy

Fixes #14.
> 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.

Correct, but they make sense in the context of their parent and in context of the branch

> 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.

That completely differs with my experience; it is not prevented at all

> Usually only one person should contribute per pull request (unless the people are in the same room and know they don't cause conflicts).

Fair enough; I've worked in a lot of situations where it's been considered totally reasonable to fix obvious mistakes in someone else's PR, but can see that not working in all scenarios

> A merge commit doesn't show the entire diff.

You may be looking for `-m` (alas I don't think it has a descriptive longopt)