Document update for vDVS upgrade instruction and expected behavior

[shuklanirdesh82]

One of our customer reached out through gitter channel, it seems our document should provide better detail about expected behavior of upgrade workflow with recommended steps.

for your reference some of the questions to address in our document:

1. how to make upgrade of vdvs a safe operation?
2. what actually happens to data which container tries to write to volume during plugin upgrade? If I run docker plugin rm command the /var/run/docker/plugins/*/vsphere.sock is deleted so how container accesses vmdk file in datastore?

For more information, please refer https://gitter.im/vmware/docker-volume-vsphere.

/CC @msterin @pdhamdhere