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 
This link will only show PRs that are have a *label* proposal. Many proposals exist that are not (yet) labeled, but *do* start with "Proposal:" (https://github.com/docker/docker/pulls?q=is%3Apr+is%3Aopen+proposal)

Not sure if GH allows this, but make the link look for both PRs with "label:proposal" or "start with proposal"

Additionally, to prevent people creating proposals that have been rejected in the past, a short snippet "also look for closed proposals"?