Documentation: ABI: Add doc for usbip-vudc attributes

As vudc provides some new attributes using sysfs infrastructure,
add a suitable documentation file for those attributes.
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-platform-usbip-vudc

+What:		/sys/devices/platform/usbip-vudc.%d/dev_desc
+Date:		April 2016
+KernelVersion:	4.6
+Contact:	Krzysztof Opasiak <k.opasiak@samsung.com>
+Description:
+		This file allows to read device descriptor of
+		gadget driver which is currently bound to this
+		controller. It is possible to read this file
+		only if gadget driver is bound, otherwise error
+		is returned.
+
+What:		/sys/devices/platform/usbip-vudc.%d/usbip_status
+Date:		April 2016
+KernelVersion:	4.6
+Contact:	Krzysztof Opasiak <k.opasiak@samsung.com>
+Description:
+		Current status of the device.
+		Allowed values:
+		1 - Device is available and can be exported
+		2 - Device is currently exported
+		3 - Fatal error occurred during communication
+		  with peer
+
+What:		/sys/devices/platform/usbip-vudc.%d/usbip_sockfd
+Date:		April 2016
+KernelVersion:	4.6
+Contact:	Krzysztof Opasiak <k.opasiak@samsung.com>
+Description:
+		This file allows to export usb device to
+		connection peer. It is done by writing to this
+		file socket fd (as a string for example "8")
+		associated with a connection to remote peer who
+		would like to use this device. It is possible to
+		close the connection by writing -1 instead of
+		socked fd.