docs(changelog): improve changelog docs

CHANGELOG.md

@@ -1,6 +1,10 @@
 # Changelog
 
+All notable changes to this project will be documented in this file.
+
 This project adheres to [Semantic Versioning][semver].
+The format of this changelog is [a variant][lib9-versionning] of [Keep a Changelog][keep-changelog].
+New entries must be placed in a section entitled `Unreleased`.
 
 ## Unreleased
 
@@ -194,4 +198,6 @@ This project adheres to [Semantic Versioning][semver].
 -   Decoders and encoders for basic types
     (bool, opaque data, floats, integers, typed arrays, UTF-8 string)
 
+[keep-changelog]: https://keepachangelog.com/en/1.0.0/
+[lib9-versionning]: https://github.com/lib9/guides/blob/main/lib9-versioning-style-guide.md#keep-a-changelog
 [semver]: https://semver.org/spec/v2.0.0.html