Add CHANGELOG.md to track version history

[Yusufolosun]

Objective
Implement a CHANGELOG.md file following Keep a Changelog format to document all notable changes to the SplitStream Protocol project.

Requirements

Structure

• Follow Keep a Changelog format
• Use Semantic Versioning
• Include sections: Added, Changed, Deprecated, Removed, Fixed, Security

Content to Document

[Unreleased]

• Current development state

[1.0.0] - 2026-01-XX (Initial Release)

• SplitStream payment splitter contract
• Comprehensive test suite (44 tests)
• Deployment scripts for Base mainnet
• Documentation (README, CONTRIBUTING, ARCHITECTURE, API, DEPLOYMENT, USAGE, FAQ, GAS_OPTIMIZATION, SECURITY_AUDIT)
• Utility scripts (checkBalance, sendPayment, releasePayment, monitor)
• Integration examples (React, Node.js, CLI, multi-contract)
• Contract verified on Basescan

Format

Changelog

All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog](https://keepachangelog.com/),
and this project adheres to [Semantic Versioning](https://semver.org/).

[Unreleased]

[1.0.0] - 2026-01-XX

Added
- Initial SplitStream contract implementation
- ...

Acceptance Criteria

• CHANGELOG.md created in project root
• Follows Keep a Changelog format
• Documents all major features and releases
• Includes proper version tags
• Links to relevant commits/PRs where applicable