add ChangeLog

Author: amartinpena

docs/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](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 5.6.0
+- Added
+  - netstandard2.0 target framework.
+
 ## 5.5.4
 - fixed
   - Fixed an issue where teh async api schema generation was not working properly with Saunter 5.5.3