Add screenshot to README.md

Author: jackycute

README.md

@@ -9,6 +9,8 @@ CodiMD
 CodiMD lets you collaborate in real-time with markdown.
 Built on [HackMD](https://hackmd.io) source code, CodiMD lets you host and control your team's content with speed and ease.
 
+![screenshot](https://raw.githubusercontent.com/hackmdio/codimd/develop/public/screenshot.png)
+
 <!-- START doctoc generated TOC please keep comment here to allow auto update -->
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 # Table of Contents