docs: Add Documentation for Mediated devices

Add file Documentation/vfio-mediated-device.txt that include details of
mediated device framework.
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Neo Jia <cjia@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/mdev/Kconfig

@@ -5,6 +5,7 @@ config VFIO_MDEV
 	default n
 	help
 	  Provides a framework to virtualize devices.
+	  See Documentation/vfio-mediated-device.txt for more details.
 
 	  If you don't know what do here, say N.