Add video demo section to README

Added a video demo section with a link to YouTube.

Author: ss0832

README.md

@@ -31,6 +31,10 @@ This program can also automatically calculate the transition-state structure fro
 - https://ss0832.github.io/
 - https://ss0832.github.io/posts/20251130_mop_usage_menschutkin_reaction_uma_en/ (In English, auto-translated)
 
+## Video Demo
+
+[![MultiOptPy Demo](https://img.youtube.com/vi/AE61iY2HZ8Y/0.jpg)](https://www.youtube.com/watch?v=AE61iY2HZ8Y)
+
 ## Features
 
 - It is intended to be used in a linux environment.