Prepare notes for 1.2.1.rc0 (#4146)
Prepare notes for 1.2.1.rc0 (#4146)

### Problem

It's been a few months since that last release of the `1.2.x` stable branch, and a few consumers have requested fixes.

### Solution

Begin to prepare a 1.2.1 release, as described in http://www.pantsbuild.org/release.html#preparation-for-the-release-from-the-stable-branch
UnrememberMe/pants,foursquare/pants,twitter/pants,benjyw/pants,15Dkatz/pants,foursquare/pants,tdyas/pants,fkorotkov/pants,twitter/pants,foursquare/pants,pombredanne/pants,tdyas/pants,mateor/pants,twitter/pants,UnrememberMe/pants,UnrememberMe/pants,landism/pants,pombredanne/pants,jsirois/pants,wisechengyi/pants,pantsbuild/pants,wisechengyi/pants,tdyas/pants,peiyuwang/pants,wisechengyi/pants,lahosken/pants,benjyw/pants,peiyuwang/pants,pombredanne/pants,tdyas/pants,wisechengyi/pants,peiyuwang/pants,pantsbuild/pants,landism/pants,baroquebobcat/pants,UnrememberMe/pants,peiyuwang/pants,15Dkatz/pants,mateor/pants,lahosken/pants,jsirois/pants,foursquare/pants,lahosken/pants,wisechengyi/pants,mateor/pants,pantsbuild/pants,UnrememberMe/pants,15Dkatz/pants,UnrememberMe/pants,benjyw/pants,landism/pants,lahosken/pants,tdyas/pants,twitter/pants,baroquebobcat/pants,lahosken/pants,pantsbuild/pants,wisechengyi/pants,landism/pants,mateor/pants,fkorotkov/pants,wisechengyi/pants,twitter/pants,tdyas/pants,pombredanne/pants,tdyas/pants,landism/pants,wisechengyi/pants,foursquare/pants,15Dkatz/pants,pantsbuild/pants,benjyw/pants,foursquare/pants,fkorotkov/pants,peiyuwang/pants,fkorotkov/pants,15Dkatz/pants,baroquebobcat/pants,foursquare/pants,pombredanne/pants,twitter/pants,15Dkatz/pants,foursquare/pants,benjyw/pants,wisechengyi/pants,baroquebobcat/pants,baroquebobcat/pants,UnrememberMe/pants,baroquebobcat/pants,pombredanne/pants,15Dkatz/pants,mateor/pants,benjyw/pants,15Dkatz/pants,fkorotkov/pants,lahosken/pants,mateor/pants,peiyuwang/pants,pantsbuild/pants,landism/pants,peiyuwang/pants,mateor/pants,tdyas/pants,twitter/pants,UnrememberMe/pants,fkorotkov/pants,landism/pants,twitter/pants,benjyw/pants,pombredanne/pants,tdyas/pants,baroquebobcat/pants,baroquebobcat/pants,baroquebobcat/pants,jsirois/pants,foursquare/pants,UnrememberMe/pants,peiyuwang/pants,lahosken/pants,fkorotkov/pants,mateor/pants,fkorotkov/pants,pantsbuild/pants,twitter/pants,lahosken/pants,landism/pants