prepare for 3.4.0 release (#355)

Author: jaeopt

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Optimizely Java X SDK Changelog
 
+## [3.4.0]
+January 27th, 2020
+
+### New Features
+- Added a new API to get a project configuration static data.
+  - Call `getOptimizelyConfig()` to get a snapshot copy of project configuration static data.
+  - It returns an `OptimizelyConfig` instance which includes a datafile revision number, all experiments, and feature flags mapped by their key values.
+  - Added caching for `getOptimizelyConfig` - `OptimizelyConfig` object will be cached and reused for the lifetime of the datafile
+  - For details, refer to a documention page: https://docs.developers.optimizely.com/full-stack/docs/optimizelyconfig-java
+
 ## 3.3.4
 December 16th, 2019