Update README to clarify snapshot support status
This PR updates README to clarify that volume snapshot supports CSI v1.0 and it is an Alpha feature in Kubernetes since 1.12 release.
# CSI Snapshotter
-The CSI external-snapshotter is part of Kubernetes implementation of [Container Storage Interface (CSI)](https://github.com/container-storage-interface/spec)
+The CSI external-snapshotter is part of Kubernetes implementation of [Container Storage Interface (CSI)](https://github.com/container-storage-interface/spec).
+
+The volume snapshot feature supports CSI v1.0 and it has been an Alpha feature in Kubernetes since v1.12.
## Overview
