doc: add demo

Author: hhow09

Diff for: README.md

@@ -1,5 +1,5 @@
 ## Coding Challenge
-Create a bot calculator using socket.io and mongoDB.
+Create a bot calculator using Typescript, socket.io and mongoDB.
 
 ## Requirements
 1. I can open the app in a browser that contains a command input
@@ -18,6 +18,10 @@ link when you’re done.
 10. Please avoid using the eval-cmd of the JS math library
 11. The more effort you put into it (e.g. design wise, documentation wise) the better the rating will be.
 
+## Demo
+![demo](./demo.gif)
+
+
 ## How to start
 1. follow the instructions in [backend/README.md](backend/README.md#run-at-local) to start backend server
 2. follow the instructions in [frontend/README.md](frontend/README.md#run-at-local) to start frontend