README.md Add info how to check the device number

Closes v4l2loopback#462

Author: abitrolly

README.md

@@ -110,9 +110,16 @@ Using `sudo` use:
 
     $ sudo modprobe v4l2loopback
 
-This will create an additional video-device, e.g. /dev/video0 (the number
-depends on whether you already had video devices on your system), which can be
-fed by various programs.
+You can check which loopback devices are created by listing contents of
+`/sys/devices/virtual/video4linux` directory. E.g. if there are two
+`v4l2loopback` devices `/dev/video0` and `/dev/video3` you would get:
+
+    $ ls -1 /sys/devices/virtual/video4linux
+    video0
+    video3
+
+These devices are ready to accept contents to show.
+
 Tested feeders:
 - GStreamer-1.0: using the  "v4l2sink" element
 - Gem(>=0.93) using the "recordV4L2" plugin