Update contributing docs

[Mr0grog]

The docs about how to contribute in README.rst and CONTRIBUTING.rst need updating (they are out of date, and also have stuff that I don't think was ever accurate 😬). The full docs in the docs/src folder (which is what ends up at https://wayback.readthedocs.io/) could also use some more info on contributing.

I also did some work yesterday to clean up the “roadmap” project and add a bunch of automation there, which should make it a useful destination for anybody trying to understand current status/what’s planned/etc. That could use a reference from all these docs as well (maybe also add a shield for it to the README?).