Add CHANGELOG

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]
+
+## [0.3.0] - 2021-12-09
+
+Initial release for mainnet.
+
+[Unreleased]: https://github.com/itchysats/itchysats/compare/0.3.0...HEAD
+[0.3.0]: https://github.com/itchysats/itchysats/compare/d12e04d4954deb2ee9ebdc9...0.3.0