thunderbolt: Add documentation how Thunderbolt bus can be used

thunderbolt: Add documentation how Thunderbolt bus can be used

Since there are no such tool yet that handles all the low-level details
of connecting devices and upgrading their firmware, add a small document
that shows how the Thunderbolt bus can be used directly from command
line.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Reviewed-by: Michael Jamet <michael.jamet@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>