Add CHANGELOG entry for intra-layer parallelism rollout

Author: nickrolfe

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 a feature in August 2023 that will improve multi-threaded performance on larger runners. [#1817](https://github.com/github/codeql-action/pull/1817)
 
 ## 2.21.2 - 28 Jul 2023