Add CHANGELOG for v0.1.0 release

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).
+
+## [0.1.0] - 2025-09-15
+
+### Added
+- Initial release of DevHub Python SDK
+- Multi-channel communication support (SMS, Email, WhatsApp, RCS)
+- Contact management functionality
+- Contact groups management
+- Sandbox environment support
+- Comprehensive test suite (185+ tests)
+- Type safety with Pydantic models
+- Full documentation with MkDocs
+
+### Features
+- **SMS**: Send messages, manage senders, buy numbers
+- **Email**: Send HTML/plain text emails with attachments
+- **WhatsApp**: Send text, template, and media messages
+- **RCS**: Rich messaging with cards, suggestions, and media
+- **Contacts**: CRUD operations, custom fields, CSV import
+- **Contact Groups**: Group management and bulk operations
+- **Authentication**: API key and sandbox API key support
+- **Error Handling**: Comprehensive exception hierarchy
+- **Validation**: Phone numbers, emails, and request validation
+
+### Technical
+- Python 3.8+ support
+- Minimal dependencies (requests, pydantic)
+- Pre-commit hooks for code quality
+- MkDocs documentation with GitHub Pages
+
+## [Unreleased]
+- Future enhancements and bug fixes will be listed here