usbip: move usbip_protocol.txt to Documentation

The usbip_protocol.txt, a document which describes usbip's
inner workings is currently located in the projects source
directory (drivers/usb/usbip/...). This patch moves it to
Documentation/usb.

This discussion was brought up by Guy Harris [0] during the
review of the USBIP dissector I wrote. For anyone interested:
support is available with the latest wireshark master/dev tree.
Simply select a packet from the usbip's tcp-stream you are
intrested on and select the USBIP as the protocol in the
"Decode As" dialog box [1].
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>

[0] <https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=12127#c2>
[1] <https://www.wireshark.org/docs/wsug_html_chunked/ChCustProtocolDissectionSection.html#ChAdvDecodeAs>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -11380,6 +11380,7 @@ M:	Valentina Manea <valentina.manea.m@gmail.com>
 M:	Shuah Khan <shuah.kh@samsung.com>
 L:	linux-usb@vger.kernel.org
 S:	Maintained
+F:	Documentation/usb/usbip_protocol.txt
 F:	drivers/usb/usbip/
 F:	tools/usb/usbip/