Improve pgindent release workflow.

Improve pgindent release workflow.

Update RELEASE_CHANGES to direct the reader towards completing the steps
outlined in the pgindent README, both as a pre-beta task and as a task
to be performed when starting a new development cycle.

This makes it less likely that somebody will miss updating the
.git-blame-ignore-revs file when running pgindent against the tree as a
routine release change task.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAH2-Wz=2PjF4As8dWECArsXxLKganYmQ-s0UeGqHHbHhqDKqeA@mail.gmail.com