web-firmware: Add Sub link to firmwares page

Tools/autotest/web-firmware/index.html

@@ -89,6 +89,8 @@ The software and hardware we provide is only for use in unmanned
 		      traditional helicopters<p>
 <a href="Rover"><img src="images/rover.png" width="80"
 		     alt="Rover">APM Rover</a> - for land vehicles and boats<p>
+<a href="Sub"><img src="images/sub.png" width="80"
+		     alt="Sub">APM Sub</a> - for ROVs and underwater vehicles<p>
 <a href="AntennaTracker"><img src="images/antenna-tracker.png" width="80"
 		     alt="AntennaTracker">APM Antenna Tracker</a> -
 		     for antenna tracking of ArduPilot vehicles<p>