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.
Could we give this a little more of a marketing twist? Maybe add something like, "This high-level overview of the course creation and delivery process introduces the basic course components and problem types available to course authors. Enroll in it now on edx.org {link to course}."