update changelog

Author: ghazi-git

docs/changelog.md

@@ -5,6 +5,8 @@ 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/).
 
 ## [UNRELEASED]
+### Fixed
+- generate the mapping for discriminator fields properly instead of showing a "null" value in the generated schema (#12).
 
 ## [0.12.0] - 2022-08-27
 ### Added