Link to documentation

It was rather surprising for me when I noticed that this package actually has docs on readthedocs.
I only looked at the repository sidebar, in the readme and the pypi page.
Therefore it would seem beneficial to add a link to the documentation in the readme and potentially also in the pypi page (https://packaging.python.org/guides/distributing-packages-using-setuptools/#project-urls)

Start by checking the README and the package's PyPI project metadata, then compare them with the existing Read the Docs documentation. Add a discoverable documentation link in the README and, if supported by the project metadata, on PyPI; done means users can find the published docs from both entry points.

Written by the indexing model from the issue text.