Add CONTRIBUTING.md

[Snailedlt]

As it stands it's really hard to figure out how to contribute to this project.

@Kludex if you're not able to maintain this project anymore, a good idea would be to add a CONTRIBUTING.md file as a last effort, so that potential maintainer candidates know what maintaining and contributing to the project entails. It'll probably lead to more interest which is good, cause it would be a shame if this project died completely

[Kludex]

Hmm... I've added the following note on the README:

#16