Need CONTRIBUTING docs

[psftw]

There are a number of subtle or unwritten conventions around what/why/how this image works. The official images documentation is quite extensive and provides context on some of the "why" and "how". The "what" is loosely covered in the published docs which is also in a separate repository. This is a lot of up-front work to get involved, and if some detail is missed it can lead to wasted efforts. We should continue to refer to the above links in our documentation here, but additional contributing guidelines covering some key ideas would go a long way.