Add document for EMC VMAX driver

The EMC VMAX driver in Juno is an enhancement from the EMC SMI-S
driver in Icehouse. In Juno, this driver only supports VMAX and
no longer supports VNX. This patch reflects this change in the driver.

Change-Id: I4c91e292b6b29774a3860289a1b83d81551e8528
Merge "Add document for EMC VMAX driver"