docs: add CHANGELOG.md (#20)

Author: CalvinAllen

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/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [0.3.0] - 2025-12-30
+
+### Added
+
+- String constant generation for top-level VSCT GUIDs
+- Implicit Microsoft.VSSDK.BuildTools package reference
+
+### Changed
+
+- Removed publish manifest generation (refactored for simplicity)
+
+## [0.2.0] - 2025-12-30
+
+### Added
+
+- Publish manifest generation and source generators
+- Analyzer reference for source generators
+
+### Fixed
+
+- Release workflow updated for moved solution location
+
+### Changed
+
+- README refreshed with emojis and reorganized sections
+- README badges updated to for-the-badge style
+
+[Unreleased]: https://github.com/CodingWithCalvin/VsixSdk/compare/v0.3.0...HEAD
+[0.3.0]: https://github.com/CodingWithCalvin/VsixSdk/compare/v0.2.0...v0.3.0
+[0.2.0]: https://github.com/CodingWithCalvin/VsixSdk/releases/tag/v0.2.0