Add draft CONTRIBUTING.md file.

CONTRIBUTING.md

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+# Contributing to Picard
+
+This document describes how to contribute to the Picard codebase. 
+It applies to code, tests, documentation, and build or CI changes.
+
+## Scope of contributions
+
+We welcome contributions that improve Picard, including:
+
+- Bug fixes
+- Performance improvements
+- New tools or tool extensions
+- Test coverage improvements
+- Documentation updates
+- Build, CI, or release-related improvements
+
+Large or behavior-changing contributions should be discussed in an issue before implementation.
+
+## Issues and discussion
+
+- Use GitHub Issues to report bugs or propose changes.
+- Include enough detail to reproduce bugs (command line, inputs, versions).
+- General usage questions should be asked on https://bioinformatics.stackexchange.com (with tag 'picard').
+
+## Development workflow
+
+1. Fork the repository.
+2. Create a topic branch from `master`.
+3. Make focused, logically separated commits.
+4. Ensure tests pass locally.
+5. Open a pull request against `master`.
+
+Pull requests should be limited in scope. 
+Large refactors or design changes may be rejected without prior discussion.
+
+## Code standards
+
+- Picard is written in Java.
+- Follow existing code style and patterns in the codebase.
+- Prefer clarity over cleverness.
+- Public APIs and command-line behavior should remain backward-compatible unless explicitly discussed.
+
+## Tests
+
+- New features and bug fixes **must** include tests.
+- Tests should fail before the fix and pass after.
+- Avoid introducing unnecessary test dependencies or large test data.
+
+## Documentation
+
+- Public-facing changes should include documentation updates.
+- Command-line behavior changes should be reflected in tool documentation.
+- Keep documentation concise and factual.
+
+## Build and verification
+
+Before submitting a pull request, run:
+
+```bash
+./gradlew test
+```
+Pull requests that fail CI will not be reviewed until fixed.
+
+## Review process
+
+All changes require review before merging.
+Maintainers may request changes for correctness, clarity, scope, or maintainability.
+Approval does not guarantee immediate merge.
+
+## Licensing
+
+By contributing to Picard, you agree that your contributions will be licensed under the project’s MIT License. 
+See LICENSE.txt for details.
+
+## Attribution
+
+GitHub automatically tracks code contributions. 
+Additional attribution requests should be discussed in an issue.