chore: add demo video to readme (#20903)

Add demo video to readme

Adds demo video to readme

README.md

@@ -15,6 +15,13 @@
   <a href="https://posthog.com/docs">Docs</a> - <a href="https://posthog.com/community">Community</a> - <a href="https://posthog.com/roadmap">Roadmap</a> - <a href="https://posthog.com/changelog">Changelog</a> - <a href="https://github.com/PostHog/posthog/issues/new?assignees=&labels=bug&template=bug_report.md">Bug reports</a> 
 </p>
 
+<p align="center">
+  <a href="https://www.youtube.com/watch?v=2jQco8hEvTI">
+    <img src="https://img.youtube.com/vi/2jQco8hEvTI/0.jpg" alt="PostHog Demonstration">
+  </a>
+  <em>See PostHog in action</em>
+</p>
+
 ## PostHog is an all-in-one, open source platform for building better products
 
 - Specify events manually, or use autocapture to get started quickly