Add minimal contributing.md file

Author: kburns

CONTRIBUTING.md

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+# Contributing to Dedalus
+
+Thank you for your interest in contributing to Dedalus! We welcome contributions in the form of bug reports, feature requests, documentation improvements, and code contributions.
+
+## Asking for Help
+- For help installing Dedalus or setting up a simulation, please use the [Google Group](https://groups.google.com/g/dedalus-users).
+- For help with source code modifications or additions, please use GitHub issues and pull requests.
+
+## Reporting Issues
+- Please search existing issues to avoid duplicates.
+- Provide a clear, minimal working example if reporting a bug.
+- Use descriptive titles and labels when possible.
+
+## Code Contributions
+- Please generally follow [PEP 8](https://peps.python.org/pep-0008/) for style.
+- Write docstrings in the [Numpy format](https://numpydoc.readthedocs.io/en/latest/format.html#parameters).
+- Try to make small commits with meaningful commit messages.
+- Ensure tests pass by running:
+  ```sh
+  python3 -m dedalus test
+  ```
+- Open a pull request with a clear description of changes.
+- Feel free to open early/draft pull requests and ask for help!
+
+## Documentation
+- Improve docs by editing `.rst` files in `docs/`.
+- Build docs locally with:
+  ```sh
+  cd docs
+  make html
+  ```
+
+We appreciate your contributions!