[docs] Missing contributing guidelines

[webknjaz]

I landed on RTD from Google (https://jwt.io/libraries?filter=python and PyPI both link the repo, so no problem there) and wanted to see how to get to this GH repo. Apparently, there's no link in the docs from this place. I just wanted to create a bug report, not even a PR, but the GH lookup experience turned out to be suboptimal.

I propose posting a link there. And while on it, it'd be nice to have a CONTRIBUTING.md that is currently missing (https://github.com/jpadilla/pyjwt/community) so that it gets shown to potential contributors on GH and could be embedded into the Sphinx docs as well (I suggest using MyST-parser to plug this one Markdown page into RST).