New changes should go into pr/ folder

This PR establishes a new workflow for documents what's new, and backwards
incompatible changes.

It will allow us to avoid extraneous merge conflicts which would otherwise come
from multiple edits to the same part of the developement.rst file directly.

I'll send a note about this to the mailing list shortly, but see 
docs/source/whatsnew/pr/REAMDME.md  for details