Update CONTRIBUTING to reflect new PR-docs process.

This updates the contributing process/docs to adjust it to reflect the new "docs first" PR process. No new proposals are being made here.

ping @shykes 
It's not clear what "ones" refers to. I take it you mean docs, so maybe something like "... that modifies relevant current docs or adds new documentation as needed." Also, as per style guide, commas are preferred over dashes!