added example for mount path permissions

content/en/docs/porch/using-porch/using-external-private-repositories.md

@@ -50,7 +50,7 @@ volumes:
 
 You may specify your desired `mountPath:` so long as the function runner can access it.
 
-Note the chosen `mountPath:` should use its own directory if placed in an existing directory so that it does not overwrite access permissions of the existing directory.
+Note the chosen `mountPath:` should use its own directory if placed in an existing directory so that it does not overwrite access permissions of the existing directory. `e.g.` if you wish to mount on `/var/tmp` you should use `mountPath: /var/tmp/<SUB_DIRECTORY>` etc.
 
 Lastly you must add the `--registry-auth-secret-path` to the function runner arguments, giving the path of the secret file mount.