Update CHANGELOG

Author: BenGraterUnity

com.unity.postprocessing/CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to this package will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+### Fixed
+- 
+
+## [3.5.1] - 2025-09-19
+
+### Added
+- Package signed
+
 ## [3.5.0] - 2025-06-18
 
 ### Added