Cleanup the README

[ericwb]

There's duplicate information in the README regarding where to contribute,
where to open bugs, where the documentation lies. This change deletes the
dups and also puts the references at the end which is standard for most
documents.

[sigmavirus24]

We might also want to add astpretty to the references for helping inspect the AST better but that's a thing we can do if we find it useful