Add contribution documentation and guidelines

[jasonspence]

To facilitate several people working on the project at once, it would be helpful to have some development "rules" that help us collaborate, and work quickly, without accidentally duplicating work, or having long and confusing discussions about who is working on what.

Task:
Create a CONTRIBUTING.md file with development documentation and guidelines.

Future:
Make sure that these guidelines are also visible in the official documentation if it makes sense to do so.