Add readme

Author: Jeroen de Graaf

README.md

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+# 🫚 Gember Event Sourcing: Dependency contracts
+[![Software License](https://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat)](LICENSE)
+[![PHP Version](https://img.shields.io/badge/php-%5E8.3-8892BF.svg?style=flat)](http://www.php.net)
+
+Dependency contracts for [gember/event-sourcing](https://github.com/GemberPHP/event-sourcing).