docs: add intro YouTube video link to README

Author: johannesjo

README.md

@@ -14,6 +14,12 @@
   <img src="https://img.shields.io/github/license/johannesjo/parallel-code" alt="License">
 </p>
 
+<p align="center">
+  <a href="https://youtu.be/sLf0tsQA3pU">
+    <img src="https://img.shields.io/badge/Watch%20Intro-YouTube-red?logo=youtube&logoColor=white&style=for-the-badge" alt="Watch intro on YouTube">
+  </a>
+</p>
+
 <p align="center">
   <img src="screens/longer-video.gif" alt="Parallel Code demo" width="800">
 </p>