docs: Add CONTRIBUTING.md

Author: Stranger6667

CONTRIBUTING.md

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+# css-inline contribution guidelines
+
+Thank you for your interest in making `css-inline` better! 
+We'd love to have your contribution. We expect all contributors to
+abide by the [Contributor Covenant Code of Conduct], which you can find in the
+[`CODE_OF_CONDUCT.md`] file in this repository.
+
+[`CODE_OF_CONDUCT.md`]: https://github.com/Stranger6667/css-inline/blob/master/CODE_OF_CONDUCT.md
+
+## License
+
+The code in this project is licensed under MIT license. 
+By contributing to `css-inline`, you agree that your contributions will be licensed under its MIT license.
+
+## Pull Requests
+
+To make changes to `css-inline`, please send in pull requests on GitHub to the `master`
+branch. We'll review them and either merge or request changes. Github Actions test
+everything as well, so you may get feedback from it too.
+
+If you make additions or other changes to a pull request, feel free to either amend
+previous commits or only add new ones, however you prefer. We may ask you to squash
+your commits before merging, depending.
+
+## Feature requests and feedback
+
+If you'd like to suggest a feature, feel free to `submit an issue <https://github.com/Stranger6667/css-inline/issues>`_
+and:
+
+* Write a simple and descriptive title to identify your suggestion.
+* Provide as many details as possible, explain your context, and how the feature should work.
+* Explain why this improvement would be useful.
+* Keep the scope narrow. It will make it easier to implement.
+
+## Report bugs
+
+Report bugs for `css-inline` in the `issue tracker <https://github.com/Stranger6667/css-inline/issues>`_.
+
+If you are reporting a bug, please:
+
+* Write a simple and descriptive title to identify the problem.
+* Describe the exact steps which reproduce the problem in as many details as possible.
+* Describe the behavior you observed after following the steps and point out the problem with that behavior.
+* Explain which behavior you expected to see instead and why.
+* Include your Rust and `css-inline` version. Additionally include your Python version if you use our Python bindings.
+
+It would be awesome if you can submit a failing test that demonstrates the problem.
+
+## Running tests
+
+Running tests requires a Flask app running in background:
+
+```
+python -m pip install flask
+python ./css-inline/tests/server.py &
+```
+
+Then run tests inside the `css-inline` directory:
+
+```
+cargo test
+```
+
+## Preferred communication language
+
+We prefer to keep all communications in English.