update changelog for new release (#322)

Author: thomaszurkan-optimizely

CHANGELOG.md

@@ -1,5 +1,18 @@
 # Optimizely Objective-C SDK Changelog
 
+## 2.1.2
+September 28, 2018
+
+This release supports xcode 10 and Swift 4. This fixes the carthage issue.
+
+### New Features
+* No new features for this patch release.
+
+### Bug Fixes:
+* Fix nullable and nonnull tags so that Swift 4 functions properly.
+* Rename protocol Optional for JSON to OPTLYOptional.
+* Fix logging of attribute as missing when included.
+
 ## 2.1.1
 September 27, 2018