Update changelog section to 0.8.4 (#161)

Author: RomanEmreis

CHANGELOG.md

@@ -5,6 +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/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/).
 
+## 0.8.4
+
+### Added
+* Open API integration (#159)
+
 ## 0.8.3
 
 ### Added