Add CHANGELOG.md to track version history

[tw4]

Summary

The repository does not have a CHANGELOG.md file. Users and contributors have no structured way to understand what changed between versions beyond reading raw git commit history.

Problems

• No clear record of breaking changes, new features, or bug fixes per version
• Users upgrading between versions have no migration guide
• Does not follow the widely adopted Keep a Changelog standard

Proposed Solution

Create a CHANGELOG.md file at the repository root following the Keep a Changelog format.

# Changelog

## [2026.3001.1] - 2026-03-01
### Added
- Content type support (JSON, FormUrlEncoded, FormData, Binary)
- Request builder fluent API
- Interceptor pattern

## [2026.2901.0] - 2026-01-29
### Changed
- Simplified generic request API (single T instead of TRequest/TResponse)

Going forward, the CHANGELOG should be updated as part of every release PR.

[Jccqt]

Hello! I would like to work on this one too if no one else is working on it. Should I based it on commits?

[tw4]

Hello, we are actually planning to take versions into account here. Once the currently open issues are resolved, we will release a new version, and this version should include the CHANGELOG.md file detailing the changes from the previous version.

[Jccqt]

I see, let me know if there's no one that will work on this, thanks.