Update CHANGELOG.md

Author: cetanu

CHANGELOG.md

@@ -5,10 +5,12 @@ 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).
 
-## [Unreleased]
+## [1.2.3] - 2020-04-15
 
 - Exclude empty lists from `to_dict` by default [#16](https://github.com/danielgtaylor/python-betterproto/pull/16)
 - Add `include_default_values` parameter for `to_dict` [#12](https://github.com/danielgtaylor/python-betterproto/pull/12)
+- Fix class names being prepended with duplicates when using protocol buffers that are nested more than once [#21](https://github.com/danielgtaylor/python-betterproto/pull/21)
+- Add support for python 3.6 [#30](https://github.com/danielgtaylor/python-betterproto/pull/30)
 
 ## [1.2.2] - 2020-01-09