Updated changelog to prepare for a release (0.4.0).

Author: m-mohr

CHANGELOG.md

@@ -4,8 +4,8 @@ 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).
 
-## [0.4.0] - Unreleased
-First version which is separated from the openEO API.
+## [0.4.0] - 2018-03-07
+First version which is separated from the openEO API. Complete rework of all processes.
 
 ## Legacy versions
 Older versions of the processes were released as part of the openEO API, see the corresponding changelog for more information.