Clarify what single-file HTML page is for

Author: gaearon

Diff for: static/html/single-file-example.html

@@ -17,5 +17,17 @@ <h1>Hello, world!</h1>,
       );
 
     </script>
+    <!--
+      Note: this page is a great way to try React but it's not suitable for production.
+      It slowly compiles JSX with Babel in the browser and uses a large development build of React.
+
+      To set up a production-ready React build environment, follow these instructions:
+      * https://reactjs.org/docs/add-react-to-a-new-app.html
+      * https://reactjs.org/docs/add-react-to-an-existing-app.html
+
+      You can also use React without JSX, in which case you can remove Babel:
+      * https://reactjs.org/docs/react-without-jsx.html
+      * https://reactjs.org/docs/cdn-links.html
+    -->
   </body>
 </html>