Create CHANGELOG.md

Author: beheshty

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).
+
+## [1.3.0] - 2025-07-13
+### Added
+- Descriptive error messages if required dependencies are missing.
+- New diagnostic IDs: `SSG001`, `SSG002`, `SSG003`.
+
+## [2.0.0] - 2025-07-16
+### Changed
+- **Breaking Change**: All generated classes are now `record` types instead of regular `class` types.
+- All properties use `init` accessors instead of `set` to support immutability.
+- Collections are now generated as `ImmutableList<T>` instead of mutable `List<T>`.
+
+### Notes
+- These changes improve immutability and align with modern C# practices.
+- This is a major version bump due to potential breaking changes in downstream usage, especially if consumers relied on mutable types or reflection-based assumptions.
+