n_gsm: add a documentation

* this documentation gives some details on how to get the n_gsm
line discipline to work with modems supporting 07.10 basic option.

* it was tested on Telit and Simcom modems.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>