add demo video

Author: apsinghdev

README.md

@@ -4,6 +4,11 @@ A real-time collaboration project to draw with your friends.
 
 ![](./drawRTC.png)
 
+<video width="600" controls autoplay>
+  <source src="./demo.mp4" type="video/mp4">
+  <!-- Your browser does not support the video tag. -->
+</video>
+
 
 ## Instructions