add link to markdown tutorial

customize.dist/messages.js

@@ -125,6 +125,7 @@ define(['/customize/languageSelector.js',
         '\n',
         '# How to use\n',
         '1. Write your slides content using markdown syntax\n',
+        '  - Learn more about markdown syntax [here](http://www.markdowntutorial.com/)\n',
         '2. Separate your slides with ---\n',
         '3. Click on the "Play" button to see the result'
     ].join('');