(chore) update changelog for release (#237)

* update changelog for release

* update for 2.1.2 release

Author: thomaszurkan-optimizely

CHANGELOG.md

@@ -1,5 +1,13 @@
 # Optimizely Android X SDK Changelog
 
+## 2.1.2
+November 8, 2018
+
+This is a patch release.
+
+### Bug Fixes
+Fix job scheduler exception when scheduling a repeatable job in the background ([#236](https://github.com/optimizely/android-sdk/pull/236)).
+
 ## 2.1.1
 October 3nd, 2018