Updated Setting up modem client under Linux (markdown)

Setting-up-modem-client-under-Linux.md

@@ -1,7 +1,8 @@
 It is possible to use modem **in client mode** with other generic Linux AX25/APRS tools on PC or Raspberry, such as [Xastir](https://xastir.org/index.php/Main_Page), use next procedure to set it up:
 - AX25 howto: http://tldp.org/HOWTO/AX25-HOWTO/
 - Install required tools: ```sudo apt-get install ax25-tools ax25-apps xastir bluez bluez-tools```
-- If you are using Bluetooth mode:
+- If you are using USB mode then `/dev/ttyUSBx` port should appear on connection
+- If you are using Bluetooth mode then you need to pair and create virtual port with `rfcomm`
   - Run bluetoothctl and pair with the modem: 
     ```
     # bluetoothctl