Create CONTRIBUTING.md (#84)

CONTRIBUTING.md

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+# ChatArena Contribution Guidelines
+
+We welcome:
+
+- Bug reports
+- Pull requests for bug fixes
+- Documentation improvements
+- Tutorials and tutorial improvements
+
+## Contributing to the codebase
+
+### Coding
+
+Contributing code is done through standard github methods:
+
+1. Fork this repo
+3. Commit your code
+4. Submit a pull request. It will be reviewed by maintainers and they'll give feedback or make requests as applicable
+
+### Considerations
+- Make sure existing tests pass (`pip install -e .[all]` and then run `pytest -v` -- if you can, use your own OpenAI key and test that the environments work successfully`)
+- Make sure your new code is properly tested and fully-covered
+- Any fixes to environments should include fixes to the appropriate documentation
+- Changes to environment functionality should be avoided when reasonable, and when they occur the environment version must be bumped.
+
+### Git hooks
+The CI will run several checks on the new code pushed to the ChatArena repository. These checks can also be run locally without waiting for the CI by following the steps below:
+1. [install `pre-commit`](https://pre-commit.com/#install),
+2. install the Git hooks by running `pre-commit install`.
+
+Once those two steps are done, the Git hooks will be run automatically at every new commit. The Git hooks can also be run manually with `pre-commit run --all-files`, and if needed they can be skipped (not recommended) with `git commit --no-verify`. **Note:** you may have to run `pre-commit run --all-files` manually a couple of times to make it pass when you commit, as each formatting tool will first format the code and fail the first time but should pass the second time.