added gif to readme, try 2

Author: mcodik

sheets/next18/README.md

@@ -11,7 +11,7 @@ Google Sheets add-on that:
 * Creates invoices in Google Docs and a sample presentation in Google Slides
   using the imported data.
 
-[[https://github.com/mcodik/apps-script-samples/blob/master/sheets/next18/demo.gif|alt=demo]]
+![Demo gif](demo.gif?raw=true "Demo")
 
 
 ## Getting started