USB: cxacru: add Documentation file

The sysfs attributes for exposing cxacru statistics/status information with
possible values is now explained in Documentation/networking/cxacru.txt
including information on the writable adsl_state attribute's commands and a
sample of the kernel log format.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Duncan Sands <duncan.sands@math.u-psud.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>