Update changelog [skip ci]

Author: JaZo

CHANGELOG.md

@@ -4,6 +4,12 @@ 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 adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Changed
+
+* Improved performance of JsonApi\Hydrator [#18](https://github.com/swisnl/json-api-client/pull/18)
+
 ## [0.7.2] - 2018-05-17
 
 ### Fixed