Add unreleased subject change to CHANGELOG

Author: camuthig

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project attempts to follow [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+* 	Allow subject to be empty/null on templated emails ([#22](https://github.com/quartzy/php-email/pull/22))
+
 ## 0.5.1 - 2018-02-25
 
 ### Added