Release notes 12-9

@mhoeber @lamagnifica @catong Please review.

@clintonb Since you're release lord (should that be capitalized?) this week, you get a sneak peek at the release notes. You don't have to do a thorough review, but feel free to read through them and let me know if anything seems amiss.
OK. I think that was my original plan, but my brain shut down (was feeling really sick last night). Thanks for the revision.