chore(CHANGELOG): switch to the keep a changelog format

CHANGELOG.md

@@ -1,3 +1,24 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+### Changed
+
+### Deprecated
+
+### Removed
+
+### Fixed
+
+### Security
+
 ## 0.28.0
 
 - chore!: upgrade dependencies ([PR 56](https://github.com/eclipse-thingweb/dart_wot/pull/56))
@@ -312,3 +333,5 @@ Various refactorings, updated README file.
 ## 0.1.0
 
 - Initial version.
+
+[unreleased]: https://github.com/eclipse-thingweb/dart_wot/compare/v0.28.0...HEAD