Adding documentation for readthedocs

[villaa]

It might be good to have our documentation on readthedocs and use sphinx to implement it. See here: https://docs.readthedocs.io/en/stable/tutorial/

The reason for this is that it will allow our documentation scale over time if necessary and is a neat place to keep the important info.

[villaa]

Maybe mkdocs is more appropriate for our needs b/c is simpler and based on markdown, as opposed to the restructured text .rst of Sphinx.

[villaa]

Also, I like the look of this paper as an example. Most detailed info can be moved to readthedocs but what remains in the README.md at top-level is a simple description, citation information, author contact, and badges....

[villaa]

Addressed by PR #38