Update CHANGELOG for next 6.1.0 release

Author: madarche

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [6.1.0] - 2021-05-03
+
+### Added
+
+- Add new "nullable" option to allow "null" additionally to any format #386 (maxwrlr)
+
+
 ## [6.0.1] - 2021-03-11
 
 ### Fixed