GTP: Add some basic documentation about drivers/net/gtp.c

In order to clarify what the module actually does, and how to use it,
let's add some basic documentation to the kernel tree, together with
pointers to related specs and projects.

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Acked-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: David S. Miller <davem@davemloft.net>