add blob contributor role in volume cloning doc

Author: umagnus

deploy/example/cloning/README.md

@@ -7,6 +7,7 @@
 ## Prerequisites
 - Make sure that the virtual network, where the driver controller pod is deployed, is added to the allowed list of the storage account's VNet setting.
 - Before proceeding, ensure that the application is not writing data to the source volume.
+- You can assign `Storage Blob Data Contributor` role to controller identity in storage account to use system-wide/user-assigned managed identity to authorize, otherwise CSI will use SAS token. If you clone a volume between different storage accounts, you can just assign `Storage Blob Data Reader` role in source storage account and `Storage Blob Data Contributor` in destination storage account.
 
 ## Create a Source PVC