added multithreaded demo

Demos.md

@@ -129,6 +129,16 @@ It highlights functionality such as:
 
 <br><hr><br>
 
+## Multithreaded Demo
+
+- `demo/multithread`: Demo for Browsers that uses multiple worker processes  
+
+- Showcases maximum performance sinch each `human` model runs in a separate worker thread  
+while main thread just combines and draws results  
+- Achieves 20+ detection frame rate and 60+ refresh frame rate on a medium hardware
+
+<br><hr><br>
+
 ## NodeJS Demo
 
 - `nodejs/node.js`: Demo using NodeJS with CommonJS module