Add a warning about the @jsx declaration

docs/docs/tutorial.md

@@ -43,6 +43,7 @@ For this tutorial we'll use prebuilt JavaScript files on a CDN. Open up your fav
       /**
        * @jsx React.DOM
        */
+      // The above declaration must remain intact at the top of the script.
       // Your code here
     </script>
   </body>