Add changelog note for C/C++ overlay

Author: IdrissRio

CHANGELOG.md

@@ -4,7 +4,7 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 ## [UNRELEASED]
 
-No user facing changes.
+- We are rolling out improved incremental analysis to C/C++ analyses that use build mode `none`. We expect this rollout to be complete by the end of April 2026.
 
 ## 4.33.0 - 16 Mar 2026