Add a CONTRIBUTING.md and refactor the README.md

[ripytide]

Should close #526

[ripytide]

I've just seen the README.md has a contributing section, perhaps the best move would be to move that information into the CONTRIBUTING.md and replace it with a link to the CONTRIBUTING.md?

[ripytide]

I've also added the remaining undocumented crate features to the README.md

[theotherphil]

This is a good idea but I want to tinker with the wording a bit so I’ll merge a jointly-authored PR with a variation on this soon.

[theotherphil]

Tweaked version merged in #640.

I'd assumed that adding a co-authored-by comment on the PR description in GitHub would add you as a co-author but it looks like not.

[ripytide]

I think you have to add a Co-authored-by: Name <email> in the commit message after a newline according to the docs to get GitHub to show both user-icons.

But it doesn't really matter, I don't mind.