usb: gadget: mass_storage: add documentation

This commit adds Documentation/usb/mass-storage.txt file.  It contains
description of how to use the mass storage gadget from user space.  It
elaborates on madule parameters and sysfs interface more then it was
written in the comments in the source code.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>