DOC: Update 1.12.0 release notes.

[charris]

[ci skip]

[charris]

I've included lists of the contributors and PRs in the release notes. That bulks them up considerably and may be somewhat controversial. Note that SciPy now does the same.

For future reference, it shoud be possible to separate the PRs by label into bugs, enhancements, etc. We may want to do that in the future if we continue down this route.

[rgommers]

That bulks them up considerably and may be somewhat controversial.

+1 from me, it's useful info. The one thing to make sure is that everyone who's merging PRs is consistent in actually setting the Milestone.

[charris]

I don't use milestones, the script searches out all merges in the specified range and extracts the relevant PR. This now works for the github squash-merge button as well as the homu delayed merges (I don't recommend the latter).

[rgommers]

Right. That's even better.