Marstek B2500: set as experimental

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/devices/marstek/MarstekB2500DeviceCoordinator.java

@@ -65,4 +65,10 @@ public class MarstekB2500DeviceCoordinator extends AbstractDeviceCoordinator {
                 R.xml.devicesettings_battery_discharge_5
         };
     }
+
+    @Override
+    public boolean isExperimental() {
+        return true;
+    }
+
 }