Add demo link

Author: arunghosh

README.md

@@ -1,5 +1,7 @@
 # react-time-line
 
+[**Demo**](https://codesandbox.io/s/react-timeline-2y11e)
+
 [![Build Status](https://travis-ci.org/arunghosh/react-time-line.svg?branch=master)](https://travis-ci.org/arunghosh/react-time-line)
 
 A react component to display event in a vertical timeline format. This is based on the timeline found in [AdminLte](https://github.com/almasaeed2010/AdminLTE).