Restore SDK documentation files (README, CHANGELOG, CONTRIBUTING, GUIDE)

These were accidentally deleted in previous commit. The SDK documentation
is essential for users to understand how to use the zeroentropy-community crate.

Author: davidatoms

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).
+
+## [0.1.1] - 2025-11-10
+
+### Changed
+- Updated repository URL in Cargo.toml
+- Improved documentation and examples
+
+## [0.1.0] - 2025-11-08
+
+### Added
+- Initial release of ZeroEntropy Rust SDK
+- Complete implementation of ZeroEntropy API endpoints
+- Collections API (create, list, delete)
+- Documents API (add text/PDF, get info, update, delete)
+- Queries API (top documents, top snippets, top pages)
+- Reranking API
+- Async client built on Tokio and reqwest
+- Configurable retry logic with exponential backoff
+- Strong typing with comprehensive error handling
+- Builder pattern for client configuration
+- Support for metadata filtering
+- PDF document support
+- Examples: basic usage and arXiv search
+- Complete API documentation
+- Apache 2.0 license
+
+[0.1.1]: https://github.com/davidatoms/zeroentropy-rust/compare/v0.1.0...v0.1.1
+[0.1.0]: https://github.com/davidatoms/zeroentropy-rust/releases/tag/v0.1.0

CONTRIBUTING.md

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+# Contributing to ZeroEntropy Rust SDK
+
+Thank you for your interest in contributing to the ZeroEntropy Rust SDK! We welcome contributions from the community.
+
+## Getting Started
+
+1. Fork the repository on GitHub
+2. Clone your fork locally
+3. Create a new branch for your feature or bugfix
+4. Make your changes
+5. Run tests to ensure everything works
+6. Submit a pull request
+
+## Development Setup
+
+```bash
+# Clone the repository
+git clone https://github.com/davidatoms/zeroentropy-rust.git
+cd zeroentropy-rust
+
+# Build the project
+cargo build
+
+# Run tests
+cargo test
+
+# Run examples (requires API key)
+export ZEROENTROPY_API_KEY="your-api-key"
+cargo run --example basic
+```
+
+## Code Style
+
+- Follow standard Rust conventions and idioms
+- Use `cargo fmt` to format your code
+- Run `cargo clippy` to catch common mistakes
+- Write clear, descriptive commit messages
+
+## Testing
+
+- Add tests for new functionality
+- Ensure all existing tests pass
+- Test your changes with real API calls when possible
+
+## Pull Request Process
+
+1. Update the README.md with details of changes if applicable
+2. Update the CHANGELOG.md with your changes
+3. Ensure your code builds and all tests pass
+4. The PR will be reviewed by maintainers
+5. Once approved, your changes will be merged
+
+## Reporting Issues
+
+When reporting issues, please include:
+
+- A clear, descriptive title
+- Steps to reproduce the issue
+- Expected behavior
+- Actual behavior
+- Your environment (OS, Rust version, SDK version)
+- Any relevant error messages or logs
+
+## Code of Conduct
+
+- Be respectful and inclusive
+- Welcome newcomers and help them learn
+- Focus on constructive feedback
+- Maintain professionalism
+
+## Questions?
+
+If you have questions about contributing, feel free to:
+
+- Open an issue for discussion
+- Contact the maintainers at founders@zeroentropy.dev
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the Apache 2.0 License.