docs: add description for contributing

[Midnighter]

• [x] fix #7

I've added some contributing guidelines to the docs. We don't have rendering of the documentation set up yet so you will have to locally install the requirements and run mkdocs serve to see them.

Some of the links don't work yet but it's more important that the general description is ready, I think.

[Midnighter]

I made another few changes mainly describing testing but also changing the order. Please give it another look.