docs: add note to changelog about using Keep a Changelog format

Author: guseggert

CHANGELOG.md

@@ -1,6 +1,13 @@
 # Changelog
 
-## v0.8.0
+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]
+
+## [0.8.0] - 2023-04-04
 ### Added
 
 - Migrated repositories into Boxo