chore: add initial CHANGELOG.md file

Introduces a changelog documenting all notable changes, releases, and contributors for the project, following Keep a Changelog and Semantic Versioning standards.

Author: jferrl

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).
+
+## [v1.2.1] - 2025-08-08
+
+### Fixed
+
+- **Security**: Fixed JWT vulnerability GO-2025-3553 by upgrading jwt dependency to v5.3.0 (#9)
+
+**Contributors**: @grinish21
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.2.0...v1.2.1>
+
+## [v1.2.0] - 2025-03-18
+
+### Changed
+
+- Bumped dependencies to latest versions (#8)
+
+**Contributors**: @candiepih (first contribution)
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.1.1...v1.2.0>
+
+## [v1.1.1] - 2024-09-09
+
+### Fixed
+
+- Fixed 404 links in README documentation (#3)
+
+### Changed
+
+- Bumped dependencies to latest versions (#6)
+- Upgraded Go version to 1.23 (#7)
+
+**Contributors**: @grinish21 (first contribution), @jferrl
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.1.0...v1.1.1>
+
+## [v1.1.0] - 2024-08-10
+
+### Added
+
+- GitHub Enterprise Server compatibility
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.0.2...v1.1.0>
+
+## [v1.0.2] - 2024-06-07
+
+### Changed
+
+- Minor improvements and bug fixes
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.0.1...v1.0.2>
+
+## [v1.0.1] - 2024-06-01
+
+### Changed
+
+- Minor improvements and bug fixes
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/compare/v1.0.0...v1.0.1>
+
+## [v1.0.0] - 2024-06-01
+
+### Added
+
+- **Initial Release**: GitHub authentication utilities for Go applications
+- **JWT Generation**: Generate JSON Web Tokens (JWT) for GitHub Apps using `NewApplicationTokenSource`
+- **Installation Tokens**: Obtain GitHub App installation tokens using `NewInstallationTokenSource`
+- **Security Compliance**:
+  - JWT expiration time limited to 10 minutes maximum
+  - Clock drift protection with 60-second buffer
+- **Configuration Options**:
+  - `WithApplicationTokenExpiration`: Customize JWT token expiration
+  - `WithHTTPClient`: Set custom HTTP client
+  - `WithInstallationTokenOptions`: Configure installation token options
+- **OAuth2 Integration**: Full compatibility with `golang.org/x/oauth2.TokenSource` interface
+
+### Documentation
+
+- Comprehensive README with usage examples
+- Integration examples with `go-github` library
+
+**Full Changelog**: <https://github.com/jferrl/go-githubauth/commits/v1.0.0>
+
+---
+
+## About This Project
+
+`go-githubauth` is a Go package that provides utilities for GitHub authentication, including generating and using GitHub App tokens and installation tokens. It implements the `TokenSource` interface from the `golang.org/x/oauth2` package for seamless integration with existing OAuth2 workflows.
+
+### Key Features
+
+- Generate GitHub Application JWT tokens
+- Obtain GitHub App installation tokens
+- RS256-signed JWTs with proper clock drift protection
+- Full OAuth2 compatibility
+- GitHub Enterprise Server support
+
+For more information, see the [README](README.md).