add Unreleased

Author: robkooper

CHANGELOG.md

@@ -4,6 +4,8 @@ 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/).
 
+## [Unreleased]
+
 ## 2.2.0 - 2019-04-03
 
 ### Changed