Updated changelog in prep for release of v2.1.2.

Author: gbmhunter

CHANGELOG.md

@@ -5,7 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [v2.1.2] - 2021-07-01
+
+- Removed download of gtest if tests are not built.
+- Install headers correctly when used as a cmake submodule (FetchContent).
+
+Big thanks to https://github.com/MadsAndreasen-Zoetis for these improvements.
 
 ## [v2.1.1] - 2021-04-05