Add comprehensive documentation and code comments

This commit adds extensive documentation to the Mistral OCR CLI project:

- Add API.md with detailed API response format documentation
- Add CHANGELOG.md to track version changes
- Add CONTRIBUTING.md with guidelines for contributors
- Enhance README.md with more detailed usage examples and troubleshooting
- Add proper docstrings to all Python modules and functions
- Update requirements.txt with development dependencies
- Improve setup.py with better metadata

These changes make the project more accessible to users and contributors.

CHANGELOG.md

@@ -0,0 +1,26 @@
+# Changelog
+
+All notable changes to the Mistral OCR CLI 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).
+
+## [Unreleased]
+
+### Added
+- Enhanced documentation including README.md, CONTRIBUTING.md, and CHANGELOG.md
+- More detailed troubleshooting section
+- API response format documentation
+
+## [0.1.0] - 2025-04-24
+
+### Added
+- Initial release of Mistral OCR CLI
+- Process command for OCR processing of PDF documents and images
+- Convert command for transforming OCR JSON to Markdown
+- Markdown command for one-step processing and conversion
+- Support for local files and URLs
+- Support for extracting and including images
+- Support for metadata extraction
+- Support for single-file and multi-file output
+- Basic error handling and retries