Add a changelog note

Author: henrymercer

CHANGELOG.md

@@ -4,7 +4,7 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 ## [UNRELEASED]
 
-No user facing changes.
+- Add a warning to help customers avoid inadvertently analyzing the same CodeQL language in multiple matrix jobs. [#1901](https://github.com/github/codeql-action/pull/1901)
 
 ## 2.21.8 - 19 Sep 2023