Update Changelog for release of version 3.0.2

Author: LaunchDarklyCI

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to the LaunchDarkly Ruby SDK will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org).
 
+## [3.0.2] - 2018-03-06
+## Fixed
+- Improved efficiency of logging by not constructing messages that won't be visible at the current log level. (Thanks, [julik](https://github.com/launchdarkly/ruby-client/pull/98)!)
+
+
 ## [3.0.1] - 2018-02-26
 ### Fixed
 - Fixed a bug that could prevent very large feature flags from being updated in streaming mode.