Update Changelog

Author: MarkBaker

CHANGELOG.md

@@ -5,7 +5,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com)
 and this project adheres to [Semantic Versioning](https://semver.org).
 
-## [Unreleased]
+## [1.5.1] - 2018-11-20
+
+### Security
+
+- Fix and improve XXE security scanning for XML-based and HTML Readers - [#771](https://github.com/PHPOffice/PhpSpreadsheet/issues/771)
 
 ### Added