Add README

Author: chr-hertel

README.md

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+# MCP PHP SDK
+
+Low-level SDK implementation of the Model Context Protocol (MCP) in PHP.
+
+This project is a collaboration between the [PHP Foundation](https://thephp.foundation/) and the
+[Symfony project](https://symfony.com/). It adopts development practices and standards from the Symfony project,
+including [Coding Standards](https://symfony.com/doc/current/contributing/code/standards.html) and the
+[Backward Compatibility Promise](https://symfony.com/doc/current/contributing/code/bc.html).
+
+Until the first major release, this SDK is considered
+[experimental](https://symfony.com/doc/current/contributing/code/experimental.html).
+
+## Installation
+
+```bash
+composer require mcp/sdk
+```
+
+## Documentation
+
+- [SDK documentation](doc/index.rst)
+- [Model Context Protocol documentation](https://modelcontextprotocol.io)
+- [Model Context Protocol specification](https://spec.modelcontextprotocol.io)
+- [Officially supported servers](https://github.com/modelcontextprotocol/servers)
+
+## Contributing
+
+We are passionate about supporting contributors of all levels of experience and would love to see you get involved in
+the project. See the [contributing guide](CONTRIBUTING.md) to get started before you
+[report issues](https://github.com/modelcontextprotocol/php-sdk/issues) and
+[send pull requests](https://github.com/modelcontextprotocol/php-sdk/pulls).
+
+## License
+
+This project is licensed under the MIT License - see the LICENSE file for details.