[w3c/editing] Update README.md (#319)

This updates the readme files to align it with the fact that this is now a working group.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/editing/pull/319

-- Commit Summary --

  * Update README.md

-- File Changes --

    M README.md (45)

-- Patch Links --

https://github.com/w3c/editing/pull/319.patch
https://github.com/w3c/editing/pull/319.diff

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/w3c/editing/pull/319