Add 'README'

README.md

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+Gon.cs
+======
+
+[![](https://github.com/lycantropos/Gon.cs/workflows/CI/badge.svg)](https://github.com/lycantropos/Gon.cs/actions/workflows/ci.yml "Github Actions")
+[![](https://img.shields.io/github/license/lycantropos/Gon.cs.svg)](https://github.com/lycantropos/Gon.cs/blob/master/LICENSE "License")
+
+### Running tests
+
+In what follows `python` is an alias for `python3.8`
+or any later version (`python3.9` and so on).
+
+Install dependencies
+```bash
+python -m pip install -r requirements-tests.txt
+```
+
+Run tests
+```bash
+pytest
+```