add heading for 3.x branch to avoid merge conflicts on bugfixes

Author: dbu

CHANGELOG.md

@@ -6,6 +6,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+# 3.x
+
 ## [3.1.3] - 2025-12-08
 
 - Fixed BC breaks accidentally introduced in 3.1.2