CHANGELOG: Prepare v0.4.0 (#38)

Author: mxinden

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).
 
-## [0.4.0] - unreleased
+## [0.4.0]
 ### Added
 - Add `global_seq` and `group_seq` to `Client`, sequence numbers assigned to test instance by the sync service. See [PR 29]