[documentation] Add "how_to_contribute" doc

[davidschlangen]

We need a page that explains how to contribute to the project. E.g., from the perspective of a student joining the project, what do they need to do? Fork the project, clone the personal fork, commit to that, submit pull request. (Then, on the side of the maintainer, what needs to be done to approve it or ask for changes?)

So, basic git stuff, but particularised for this project and not making too many assumptions about what contributor already knows.

Any takers?