Update CHANGELOG to follow the "Keep a Changelog" format

Author: Frederick Pringle

CHANGELOG.md

@@ -1,5 +1,15 @@
-# Revision history for generic-diff
+# Changelog
 
-## 0.1.0.0 -- 2025-2-25
+All notable changes to `generic-diff` will be documented in this file.
 
-* First version. Released on an unsuspecting world.
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Haskell Package Versioning Policy](https://pvp.haskell.org).
+
+## [Unreleased]
+
+### Added
+
+- First version. Released on an unsuspecting world.
+- Let users extend the built-in diff types with custom diffs via the `SpecialDiff` class in [#9](https://github.com/fpringle/generic-diff/pull/9).
+
+[unreleased]: https://github.com/fpringle/generic-diff/compare/74b5028...HEAD