Add minimal CONTRIBUTING.txt with guidelines #16

Since #12 is the first contribution from somebody not in AUTHORS.txt: What kind of policy should we follow? I would recommend the following approach: A contributor of a PR can ask for being added to AUTHORS.txt (may also be added directly as part of the PR). Generally, I would be generous with adding people to the list of AUTHORS.txt if they ask because this also incentivizes small contributions (fixing typos etc.); and we do not have to start discussing which contribution is considered as relevant enough for being added to AUTHORS.txt.

However, I would not start checking this automatically or promising that this will be 100% consistent with the actual contributors. It's only updated on demand.

@wardvp does this make sense?