Add beta server link to ask users to try out the beta features

templates/index.html

@@ -33,7 +33,7 @@
 
     <div id="menu">
         <h2>ArduPilot Custom Firmware Builder</h2>
-
+        <p style="color: red;">To try out the newest features of the application, visit our beta server at <a href="https://custom-beta.ardupilot.org">custom-beta.ardupilot.org</a></p>
         <form action="/generate" method="post">
             <div id="vehicle_list">
                 <label for="vehicle">Choose a vehicle: