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.

Author: xing-yang

README.md

@@ -1,6 +1,8 @@
 # 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