Docs: Keep unreleased section in changelog

Author: schmika

CHANGELOG.md

@@ -5,6 +5,8 @@ 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).
 
+## Unreleased
+
 ## [7.0.0](https://github.com/dbmdz/xml-toolbox/releases/tag/7.0.0) - 2025-08-05
 
 ### Changed