Update changelog

com.unity.render-pipelines.high-definition/CHANGELOG.md

 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).
 
+## [3.4.0-preview]
+
+### Fixed
+- Stencil test during decals normal buffer update is now properly applied
+
 ## [3.3.0-preview]
 
 ### Added