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.
I don't think you need that, but an invitation to enroll, or info that it only takes an hour or two to take would be nice.