Create CONTRIBUTING.md

CONTRIBUTING.md

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+We welcome any contributions via pull requests. We are especially interested in a collaborator that would either backport xphyle to Python 2.x or implement a compatibility layer to my xphyle Python version-independent.
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+All code must be written in idiomatic python 3. Note that we use [PEP484](https://www.python.org/dev/peps/pep-0484/) type hints. Variable annotations are
+defined where needed using the comment syntax. Static code analysis is performed usying [mypy](http://mypy-lang.org/) and pylint.
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+Style-wise, we try to adhere to the Google python style guidelines. We use Google-style docstrings, which are formatted by the [Napoleon Sphinx Plugin](https://pypi.python.org/pypi/sphinxcontrib-napoleon). 
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+We enforce the [Contributor Covenant](http://contributor-covenant.org/) code of conduct.