Update Changelog for release of version 3.0.0

Author: LaunchDarklyCI

CHANGELOG.md

@@ -2,6 +2,14 @@
 
 All notable changes to the LaunchDarkly Ruby SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [3.0.0] - 2018-02-22
+### Added
+- Support for a new LaunchDarkly feature: reusable user segments.
+
+### Changed
+- The feature store interface has been changed to support user segment data as well as feature flags. Existing code that uses `InMemoryFeatureStore` or `RedisFeatureStore` should work as before, but custom feature store implementations will need to be updated.
+
+
 ## [2.5.0] - 2018-02-12
 
 ## Added