Add CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+Pull requests are always welcome, but before you contribute to this project, please be mindful of the following guidelines:
+
+- This project adheres to [PEP8](https://www.python.org/dev/peps/pep-0008/) and most of [PEP257](https://www.python.org/dev/peps/pep-0257/) conventions. The [`pylama`](https://github.com/klen/pylama) linter is used for checking the code. The full configuration can be found in the `setup.cfg` file. Please read these conventions and the project's configuration.
+
+- The [`isort`](https://pypi.python.org/pypi/isort) package is used for sorting imports. The full configuration can be found in `setup.cfg`. Please review them.