Mention semantic versioning in changelog

Author: sipa

CHANGELOG.md

@@ -1,6 +1,9 @@
 # Changelog
 
-The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
+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]