Add Contributing Guidelines and Code of Conduct

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

CONTRIBUTING.md

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+# Contributing to Model Context Protocol Java SDK
+
+Thank you for your interest in contributing to the Model Context Protocol Java SDK!
+This document outlines how to contribute to this project.
+
+## Prerequisites
+
+The following software is required to work on the codebase:
+
+- `Java 17` or above
+- `Docker`
+- `npx`
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork:
+
+```bash
+git clone https://github.com/YOUR-USERNAME/java-sdk.git
+cd java-sdk
+```
+
+3. Build from source:
+
+```bash
+./mvnw clean install -DskipTests # skip the tests
+./mvnw test # run tests
+```
+
+## Reporting Issues
+
+Please create an issue in the repository if you discover a bug or would like to 
+propose an enhancement. Bug reports should have a reproducer in the form of a code 
+sample or a repository attached that the maintainers or contributors can work with to 
+address the problem.
+
+## Making Changes
+
+1. Create a new branch:
+
+```bash
+git checkout -b feature/your-feature-name
+```
+
+2. Make your changes
+3. Validate your changes:
+
+```bash
+./mvnw clean test
+```
+
+### Change Proposal Guidelines
+
+#### Principles of MCP
+
+1. **Simple + Minimal**: It is much easier to add things to the codebase than it is to
+   remove them. To maintain simplicity, we keep a high bar for adding new concepts and
+   primitives as each addition requires maintenance and compatibility consideration.
+2. **Concrete**: Code changes need to be based on specific usage and implementation
+   challenges and not on speculative ideas. Most importantly, the SDK is meant to 
+   implement the MCP specification.
+
+## Submitting Changes
+
+1. For non-trivial changes, please clarify with the maintainers in an issue whether 
+   you can contribute the change and the desired scope of the change.
+2. For trivial changes (for example a couple of lines or documentation changes) there 
+   is no need to open an issue first.
+3. Push your changes to your fork.
+4. Submit a pull request to the main repository.
+5. Follow the pull request template.
+6. Wait for review.
+
+## Code of Conduct
+
+This project follows a Code of Conduct. Please review it in
+[CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md).
+
+## Questions
+
+If you have questions, please create a discussion in the repository.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the MIT
+License.
+
+## Security
+
+Please review our [Security Policy](SECURITY.md) for reporting security issues.