Add release notes for 1.2.1

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[bob-carpenter]

I was a bit unclear on the language in the first comment. Otherwise, looks good.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (46f8608) 79.82% compared to head (c12b9dd) 79.88%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #729      +/-   ##
===========================================
+ Coverage    79.82%   79.88%   +0.06%     
===========================================
  Files           75       50      -25     
  Lines        11502     7666    -3836     
===========================================
- Hits          9181     6124    -3057     
+ Misses        2321     1542     -779     

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[bob-carpenter]

That's much clearer. Thanks!

[WardBrian]

@mitzimorris I think it would be good to release this week if you have a chance to look over the release notes. Thank you!

[mitzimorris]

LGreatTM!