Update custom-elements changelog

Author: startracex

packages/custom-elements/CHANGELOG.md

@@ -5,7 +5,9 @@ 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 -->
+## Unreleased
+
+- Re implement the CustomElementRegistry interface ([#606](https://github.com/webcomponents/polyfills/pull/606))
 
 ## [1.6.0] - 2023-03-30