docs: link the explainer video from the README

A thumbnail under the badges links to the 2½-minute overview on
YouTube. The image is referenced by its raw.githubusercontent URL so it
also renders on PyPI, whose README renderer strips video embeds.

The video's source lives in its own repo (vgi-python-video).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: rustyconover

README.md

@@ -16,6 +16,11 @@
   <a href="LICENSE"><img src="https://img.shields.io/pypi/l/vgi-python" alt="License"></a>
 </p>
 
+<p align="center">
+  <a href="https://youtu.be/J2E51PTZn6o"><img src="https://raw.githubusercontent.com/Query-farm/vgi-python/main/docs/assets/video-thumbnail.jpg" alt="Watch: VGI explained in 2½ minutes, on YouTube" width="640"></a><br>
+  <sub>▶ <a href="https://youtu.be/J2E51PTZn6o">Watch the 2½-minute overview</a></sub>
+</p>
+
 ---
 
 ## See It in Action