docs: add note about `DEVELOPMENT_MAINTENANCE`

In the instructions for developing a new package, we had no mention of creating a DEVELOPMENT_MAINTENANCE file. This MR adds a callout to create the file in the ./docs dir and what the file should include. As discussed during Value Stream time at cohort.

changed milestone to %2.23.0

added kinddocs label

requested review from @ortiz.jacob, @andrewshoell, @ryan.j.garcia, @chris.oconnell, @michaelmartin, and @ryan.thompson.44

assigned to @noahbirrer

added statusreview label

lgtm

approved this merge request

approved this merge request

merged

mentioned in commit 3a3fe31c