Document release reset of master. (#5397)
Document release reset of master. (#5397)

Make it clear that releases can be performed in the face of rapid
changes hitting master by documenting the pre-release step of getting
the releaser's local clone set to the release commit.
tdyas/pants,jsirois/pants,twitter/pants,UnrememberMe/pants,foursquare/pants,UnrememberMe/pants,baroquebobcat/pants,foursquare/pants,benjyw/pants,pantsbuild/pants,UnrememberMe/pants,pantsbuild/pants,jsirois/pants,benjyw/pants,twitter/pants,benjyw/pants,wisechengyi/pants,pantsbuild/pants,foursquare/pants,wisechengyi/pants,baroquebobcat/pants,benjyw/pants,baroquebobcat/pants,tdyas/pants,baroquebobcat/pants,wisechengyi/pants,benjyw/pants,baroquebobcat/pants,UnrememberMe/pants,foursquare/pants,baroquebobcat/pants,twitter/pants,foursquare/pants,foursquare/pants,benjyw/pants,tdyas/pants,tdyas/pants,twitter/pants,foursquare/pants,wisechengyi/pants,tdyas/pants,foursquare/pants,UnrememberMe/pants,jsirois/pants,pantsbuild/pants,tdyas/pants,twitter/pants,twitter/pants,UnrememberMe/pants,wisechengyi/pants,UnrememberMe/pants,wisechengyi/pants,UnrememberMe/pants,tdyas/pants,twitter/pants,baroquebobcat/pants,baroquebobcat/pants,twitter/pants,benjyw/pants,baroquebobcat/pants,foursquare/pants,wisechengyi/pants,tdyas/pants,wisechengyi/pants,pantsbuild/pants,wisechengyi/pants,pantsbuild/pants,tdyas/pants,pantsbuild/pants,UnrememberMe/pants,twitter/pants