Add CI badge

Author: Maxime Rainville

README.md

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 # PSR-14 Async Event Dispatcher (Experimental)
 
+[![CI](https://github.com/archipro/revolt-event-dispatcher/actions/workflows/ci.yml/badge.svg)](https://github.com/archipro/revolt-event-dispatcher/actions/workflows/ci.yml)
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 A PSR-14 compatible event dispatcher implementation using Revolt and AMPHP for asynchronous event handling.
 
 ## Features