lustre: Add some basic documentation

This adds drivers/staging/lustre/README.txt with some
basic information about Lustre and how to use it.

Eventually this will need to move to Documentation/filesystems/lustre.txt

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>