add demo

README.md

@@ -17,6 +17,10 @@ Example:
 	pert.sh "1,2,3" "15,17,20"
 </pre>
 
+### Demo
+
+![pert example](https://cloud.githubusercontent.com/assets/6382002/13582789/8205bac0-e4ae-11e5-9a03-894e32943f30.gif)
+
 ### Example
 
 Command: