added needed permissions to Disk Image Builder ReadMe (#894)

added needed permissions to docs

tools/gke-disk-image-builder/README.md

@@ -24,7 +24,7 @@ examples below to understand how this tool work better.
 1.  Compute Engine API must be enabled.
     (https://console.developers.google.com/apis/api/compute.googleapis.com/overview?project=$PROJECT_NAME)
 1.  Verify that `$PROJECT_NUMBER-compute@developer.gserviceaccount.com` has
-    `storage.objectCreator` permissions to the provided *GCS path* for the logs.
+    `storage.objectCreator` and `storage.objectViewer` permissions to the provided *GCS path* for the logs.
     You can run the following command to grant proper permissions for this:
 
     ```shell