Prepare changelog for 0.8 release

Author: Mario Blazek

doc/CHANGELOG.md

@@ -6,7 +6,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
-## [Unreleased]
+## [0.8]
 ### Added
 - documentation update.
 - every action throws ActionFailedException on failure.