Changelog updated for version 0.5.1

Author: remorhaz

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased]
+## [0.5.1] - 2019-02-17
 ### Added
 - Upgraded `remorhaz/php-json-pointer`.