Update Python version support and install docs in README

[rgommers]

Closes gh-467

[rdbisme]

Nice. There's a related issue to this: I never managed to get the hands on the RTD account. It still works, but I don't control it. Any experience on what we might do to get back access to it?

[rgommers]

Nice. There's a related issue to this: I never managed to get the hands on the RTD account. It still works, but I don't control it. Any experience on what we might do to get back access to it?

I don't have experience with that, aside from migrating away from RTD to GitHub Pages. https://docs.readthedocs.com/platform/stable/abandoned-projects.html#reachability doesn't look like there's a process for this, aside from getting in touch with the current owner of the RTD login credentials for bottleneck. I assume you already tried that and got no response? If so, it may be worth asking RTD site support, since it's a little odd that this question isn't answered in their docs (unless I missed it of course).