Update CHANGELOG.md

Author: oleibman

CHANGELOG.md

@@ -36,6 +36,7 @@ and this project adheres to [Semantic Versioning](https://semver.org). Thia is a
 - Php8.5 deprecates use of null as array index. [PR #4634](https://github.com/PHPOffice/PhpSpreadsheet/pull/4634)
 - For Php8.5, replace one of our two uses of `__wakeup` with `__unserialize`, and eliminate the other. [PR #4639](https://github.com/PHPOffice/PhpSpreadsheet/pull/4639)
 - Use prefix _xlfn for BASE function. [Issue #4638](https://github.com/PHPOffice/PhpSpreadsheet/issues/4638) [PR #4641](https://github.com/PHPOffice/PhpSpreadsheet/pull/4641)
+- Additional support for union and intersection. [PR #4596](https://github.com/PHPOffice/PhpSpreadsheet/pull/4596)
 
 ## 2025-09-03 - 5.1.0