add comment for tx interval

ST-B-L072Z-LRWAN1/longfi-us915-cayenne-sensor/longfi-us915-cayenne-sensor.ino

@@ -128,5 +128,5 @@ void loop( void )
     LoRaWAN.sendPacket(1, lpp.getBuffer(), lpp.getSize());
   }
 
-  delay(20000);
+  delay(20000); //20 Seconds
 }

ST-B-L072Z-LRWAN1/longfi-us915/longfi-us915.ino

@@ -52,5 +52,5 @@ void loop( void )
         LoRaWAN.sendPacket(1, payload, sizeof(payload));
     }
 
-    delay(20000);
+    delay(20000); //20 Seconds
 }