update changelog

Author: TomJGooding

CHANGELOG.md

@@ -5,6 +5,12 @@ 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/).
 
+## Unreleased
+
+### Added
+
+- `Enter` and `Leave` events can now be used with the `on` decorator https://github.com/Textualize/textual/pull/5159
+
 ## [0.84.0] - 2024-10-22
 
 ### Fixed