Update README.md to note external-snapshotter is required along with …

…explaining how the snapshot is created

deploy/example/snapshot/README.md

@@ -1,6 +1,9 @@
 # Volume Snapshot Example
 
+This creates a snapshot of a volume using `tar`.
+
 - supported from v4.3.0
+- Make sure you have `externalSnapshotter.enabled=true` if you are using the Helm chart.
 
 ## Create source PVC and an example pod to write data 
 
@@ -73,4 +76,4 @@ outfile
 ```
 
 ### Links
- - [CSI Snapshotter](https://github.com/kubernetes-csi/external-snapshotter)
+ - [CSI Snapshotter](https://github.com/kubernetes-csi/external-snapshotter)