updated changelog for v0.5.1 release

CHANGELOG.md

@@ -5,6 +5,16 @@ 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).
 
+## [0.5.1] - 2017-10-23
+
+### Changed
+
+- OPAL-RT AsyncIP client is only build if libopal submodule is present
+
+### Fixed
+
+- Build warnings and errors on Ubuntu 16.04
+
 ## [0.5.0] - 2017-10-18
 
 ### Added