Add description of Storage Class Parameters to README.
Add description of Storage Class Parameters to README.

Signed-off-by: Humble Chirammal <dc8ba7e82a7df22725dedec9b10c7aaab9ec713f@redhat.com>

humblec/external-storage,virtuozzo/external-storage,childsb/external-storage,humblec/external-storage,kubernetes-incubator/external-storage,aburluka/external-storage,childsb/external-storage,virtuozzo/external-storage,dhirajh/external-storage,aburluka/external-storage,dhirajh/external-storage,aburluka/external-storage,kubernetes-incubator/external-storage,dhirajh/external-storage,kubernetes-incubator/external-storage,aburluka/external-storage,virtuozzo/external-storage,dhirajh/external-storage,childsb/external-storage,humblec/external-storage