DOC: Updating contributing docs

Also adds a stub for the 0.6. release notes so we can ask people to fill in as we go this time.

Would be good to get another set of eyes on the minimum requirements for accepting a PR. Anything else?

Hopefully, this ups the ante a bit for getting PRs merged -- mainly focusing more on documentation and editing the release notes. This will improve the quality of the code base and make it easier to just tag and release when ready.