Add a section for transitioning from old to new design

Add a section to explain how to transition from old to new
snapshot support.

contributors/design-proposals/storage/csi-snapshot.md

@@ -324,3 +324,57 @@ CreateSnapshot is a synchronous function and it must be blocking until the snaps
 Create volume from snapshot will be handled by the CreateVolume controller function in the CSI Volume Driver.
 
 Refer to [Container Storage Interface (CSI)](https://github.com/container-storage-interface/spec) for detailed instructions on how CSI Volume Driver shall implement snapshot functions.
+
+
+## Transition to the New Snapshot Support
+
+To use the snapshot implementation in [external storage repo](https://github.com/kubernetes-incubator/external-storage/tree/master/snapshot), an external snapshot controller and external provisioner need to bedeployed.
+
+To use the new snapshot implementation for CSI, a sidecar container for the external snapshot controller needs to be deployed in addition to the sidecar container for the external provisioner.
+
+The differences between the two are as follows from a user's point of view:
+
+New Design
+* Supports CSI volume drivers.
+* To create a snapshot for CSI, a SnapshotClass can be created and specified in the spec of VolumeSnapshot.
+* To restore a volume from the snapshot, user should use the same StorageClass that is used for the original PVC.
+
+Old Design
+* Does not support CSI volume drivers.
+* SnapshotClass is not needed to create a snapshot and this concept does not exist in the old design.
+* To restore a volume from the snapshot, however, user needs to create a new StorageClass that is different from the original one for the PVC.
+
+Here is an example yaml file to create a snapshot in the old design:
+
+```GO
+
+apiVersion: volumesnapshot.external-storage.k8s.io/v1
+kind: VolumeSnapshot
+metadata:
+  name: hostpath-test-snapshot
+spec:
+  persistentVolumeClaimName: pvc-test-hostpath
+
+```
+
+Here is an example to create a SnapshotClass and create a snapshot in the new design:
+
+```GO
+
+apiVersion: volumesnapshot.csi.k8s.io/v1alpha1
+kind: SnapshotClass
+metadata:
+  name: csi-hostpath-snapclass
+  snapshotter: csi-hostpath
+
+apiVersion: volumesnapshot.csi.k8s.io/v1alpha1
+kind: VolumeSnapshot
+metadata:
+  name: snapshot-demo
+spec:
+  snapshotClassName: csi-hostpath-snapclass
+  persistentVolumeClaimName: hpvc
+
+```
+
+To transition from the old to the new snapshot support, user needs to deploy the CSI external snapshot controller as a side car container along with the external provisioner and use the new definitions for the SnapshotClass, VolumeSnapshot, and PVC yaml files.