add demo site to readme

Author: MandeeD

README.md

@@ -5,7 +5,9 @@ The purpose of this presentation is to teach:
 - When to use CSS Grid and Flexbox
 - How to make basic layouts with CSS Grid and Flexbox
 
-### Basic setup
+Demo: https://mandeed.github.io/css-grid-presentation/
+
+### Basic local setup
 
 The core of reveal.js is very easy to install. You'll simply need to download a copy of this repository and open the index.html file directly in your browser.