Add changelog

Author: oli-obk

CHANGELOG.md

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+# 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/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+* Started maintaining a changelog
+
+### Fixed
+
+### Changed
+
+### Removed
+
+* `$DIR` and `RUSTLIB` replacements
+
+## [0.21.2] - 2023-09-27