i2400m: documentation and instructions for usage

The driver for the i2400m is a stacked driver. There is a core driver,
the bus-generic driver that has no knowledge or dependencies on how
the device is connected to the system; it only knows how to speak the
device protocol. Then there are the bus-specific drivers (for USB and
SDIO) that provide backends for the generic driver to communicate with
the device.

The bus generic driver connects to the network and WiMAX stacks on the
top side, and on the bottom to the bus-specific drivers.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>