First-draft release notes for back-branch releases

I've made a pass over the commit log up to now, and prepared draft
release note entries for everything that seemed worth documenting;
see

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=65a82a7649860f8010db581a0d1f12aa92f5969b

As I usually do, I dropped all of these into a section for 11.1,
although some won't remain there because they don't apply to the
v11 branch or were committed in time for 11.0.  I'll sort that
out on Sunday.

Comments please ...

            regards, tom lane