Add CONTRIBUTING file

CONTRIBUTING

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+# Contributing to JAXsim :rocket:
+
+Hello Contributor,
+
+We're thrilled that you're considering contributing to JAXsim! Here's a brief guide to help you seamlessly become a part of our project.
+
+## Development Environment :hammer_and_wrench:
+
+Make sure your development environment is set up. Follow the installation instructions in the [README](./README.md) to get JAXsim and its dependencies up and running.
+
+## Branching Strategy :deciduous_tree:
+
+When making changes, create a new branch from main. Use a descriptive name like `feature/your-feature` or `fix/your-fix`. This helps us understand the purpose of the branch.
+
+## Making Changes :construction:
+
+Before submitting a pull request, create an issue to discuss your changes if major changes are involved. This helps us understand your needs and provide feedback. Clearly describe your pull request, referencing any related issues. Follow the [PEP 8](https://peps.python.org/pep-0008/) style guide and include relevant tests.
+
+## Testing :test_tube:
+
+Your code will be tested with the CI/CD pipeline before merging. Feel free to add new ones or update the existing tests in the [workflows](./github/workflows) folder to cover your changes.
+
+## Documentation :book:
+
+Update the documentation in the [docs](./docs) folder and the [README](./README.md) to reflect your changes, if necessary.
+
+## Code Review :eyes:
+
+Expect feedback during the code review process. Address comments and make necessary changes. This collaboration ensures quality.
+
+## License :scroll:
+
+JAXsim is under the [BSD 3-Clause License](./LICENSE). By contributing, you agree to the same license.
+
+Thank you for contributing to JAXsim! Your efforts are appreciated.