Add "application/vnd.api+json" support: update CHANGELOG.md

Author: pongee

CHANGELOG.md

@@ -4,6 +4,10 @@ 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).
 
+## [5.7.0] - 2021-05-28
+### Added
+- Support for `application/vnd.api+json` MIME type.
+
 ## [5.6.0] - 2021-05-10
 ### Added
 - Api Client Generator version to README generation