Add "Misc/NEWS.d" directory tree for "blurb".

CPython workflow is changing!  We're going to start using "blurb"
to manage Misc/NEWS entries:
    https://github.com/python/core-workflow
(This will be a big win for release managers, honest.)

This checkin simply populates the "Misc/NEWS.d" subdirectory tree
so that people can start putting their news entries in there.
No other changes (yet).