Rewrite main README

Created by: Zaharid

The main README should now be public facing, tidy and mainly link to the docs. Because of the docs, I don't think it needs to be very long but it needs to mention at least:

• Citation policy.
• The fact that we only "support" conda (quotes meaning that even for conda we are only willing to help so much).
• Contribution policy (which sort of implies that it should be public, contrary to the current version of #1303).