Add readme to the docs directory

[asbjornu]

This PR adds a readme.md file to the docs directory, to make it easier contributing to the documentation, serve the documentation up locally with mkdocs serve, etc.

[pascalberger]

Thanks @asbjornu! I would suggest that we also add links to the new readme to make it more discoverable. Eg to CONTRIBUTING.md

[asbjornu]

@pascalberger Good idea. Let me test a few ways of how it might best work. Potentially breaking commits incoming! 😄

[asbjornu]

@pascalberger There, I think that looks ok. Thoughts?

[gep13]

LGTM! 👍