First-draft back-branch release notes are up

[Tom Lane]

Much less exciting than the v18 release notes, but we
still gotta do 'em.  See

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=176877f461a8b55e921f597fb217f6ab89ee019f

As usual, please send corrections by Sunday.

                        regards, tom lane