transport: Log it explicitly if user doesn't specify a reader

Change-Id: I37e9d62fabf237ece7e49d8f2253c606999d3d02
diff --git a/pySim/transport/__init__.py b/pySim/transport/__init__.py
index 6cccf8c..e90ced6 100644
--- a/pySim/transport/__init__.py
+++ b/pySim/transport/__init__.py
@@ -297,6 +297,7 @@
         sl = ModemATCommandLink(
              device=opts.modem_dev, baudrate=opts.modem_baud, **kwargs)
     else:  # Serial reader is default
+        print("No reader/driver specified; falling back to default (Serial reader)")
         from pySim.transport.serial import SerialSimLink
         sl = SerialSimLink(device=opts.device,
                            baudrate=opts.baudrate, **kwargs)