docs: add CI status badge to README

Author: alexisbouchez

README.md

@@ -3,6 +3,9 @@
 > Run containers as microVMs. Fast, secure, simple.
 
 <div align="center">
+  <a href="https://github.com/valyentdev/ravel/actions/workflows/ci.yml">
+    <img src="https://github.com/valyentdev/ravel/actions/workflows/ci.yml/badge.svg" alt="CI" />
+  </a>
   <a href="https://discord.gg/HXkpCG7DEH">
     <img src="https://img.shields.io/badge/chat-on%20discord-7289DA.svg" alt="Discord Chat" />
   </a>