Add CONTRIBUTING.md (#85)

Author: msaroufim

CONTRIBUTING.md

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+# Contributing to BackendBench
+
+## License
+
+BackendBench is BSD-3-Clause licensed, as found in the LICENSE file.
+
+## Our Development Process
+
+BackendBench is actively developed internally at Meta and synced to GitHub regularly. External contributions are welcomed and will be reviewed by the Meta team.
+
+## Code Quality
+
+We use [ruff](https://docs.astral.sh/ruff/) for linting and code formatting.
+
+## Pre-commit Hooks
+
+To make development easier, we provide pre-commit hooks that automatically run ruff on your changes:
+
+```bash
+pip install pre-commit
+pre-commit install
+```
+
+This will automatically lint your code before each commit, ensuring consistent code quality across the project.
+
+## Pull Requests
+
+We actively welcome your pull requests.
+
+1. Fork the repo and create your branch from `main`.
+2. If you've added code that should be tested, add tests.
+3. If you've changed APIs, update the documentation.
+4. Ensure the test suite passes.
+5. Make sure your code lints.
+6. If you haven't already, complete the Contributor License Agreement ("CLA").
+
+## Contributor License Agreement ("CLA")
+
+In order to accept your pull request, we need you to submit a CLA. You only need
+to do this once to work on any of Meta's open source projects.
+
+Complete your CLA here: <https://code.facebook.com/cla>
+
+## Issues
+
+We use GitHub issues to track public bugs. Please ensure your description is
+clear and has sufficient instructions to be able to reproduce the issue.