Update changelog

Author: MattiasBuelens

CHANGELOG.md

@@ -10,6 +10,10 @@
 > -   🏠 Internal
 > -   💅 Polish
 
+## Unreleased
+
+-   🚀 Added a polyfill bundle in `@theoplayer/web-ui/polyfills`. This contains polyfills for ES2015 features (e.g. `Promise`) and Custom Elements.
+
 ## v1.4.0 (2023-10-04)
 
 -   💥 **Breaking Change**: This project now requires THEOplayer version 6.0.0 or higher.