Update snapshot example doc
Adding the comments in snapshot example for installing the driver in dev
version.
@@ -102,6 +102,8 @@ web-server 1/1 Running 0 1m
### Snapshot Example
+Please note that VolumeSnapshot is currently alpha feature. In order to enable this feature, please install CSI driver with
+dev version by setting the environment variable "GCE_PD_DRIVER_VERSION=dev"
1. Create example Default Snapshot Class
```
$ kubectl create -f ./examples/kubernetes/demo-defaultsnapshotclass.yaml
