chore(release): Preparing for 3.1.0 release (#294)

Author: aliabbasrizvi

Diff for: CHANGELOG.md

@@ -1,5 +1,22 @@
 # Optimizely Java X SDK Changelog
 
+## 3.1.0
+May 6th, 2019
+
+### New Features:
+- Introduced Decision notification listener to be able to record:
+    - Variation assignments for users activated in an experiment.
+    - Feature access for users.
+    - Feature variable value for users.
+- Added APIs to be able to conveniently add Decision notification handler (`addDecisionNotificationHandler`) and Track notification handler (`addTrackNotificationHandler`).
+
+### Bug Fixes:
+- Feature variable APIs return default variable value when featureEnabled property is false. ([#274](https://github.com/optimizely/java-sdk/pull/274))
+
+### Deprecated
+- Activate notification listener is deprecated as of this release. Recommendation is to use the new Decision notification listener. Activate notification listener will be removed in the next major release.
+- `addActivateNotificationListener`, `addTrackNotificationListener` and `addNotificationListener` APIs on `NotificationCenter`.
+
 ## 3.0.1
 
 April 23, 2019