Add 3.1.0 changelog

Author: IreneStr

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/) 
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## 3.1.0
+
+### Fixed
+- Fixes a bug where a 'You're using WordPress in a language we don't support yet.' notice would be shown for formal and informal locales when the locale actually was supported.
+
 ## 3.0.0
 
 ### Added