Add changelog note

Author: henrymercer

CHANGELOG.md

@@ -6,7 +6,7 @@ Note that the only difference between `v2` and `v3` of the CodeQL Action is the
 
 ## [UNRELEASED]
 
-No user facing changes.
+- We are rolling out a feature in September/October 2024 that sets up CodeQL using a bundle compressed with [Zstandard](http://facebook.github.io/zstd/). Our aim is to improve the performance of setting up CodeQL. [#2502](https://github.com/github/codeql-action/pull/2502)
 
 ## 3.26.8 - 19 Sep 2024