Update mirror.md

It was not obvious that additional configuration is needed for the official Registry image.

cc @dmcgowan @stevvooe

Author: AkihiroSuda

registry/recipes/mirror.md

@@ -59,6 +59,8 @@ be configured to use the `filesystem` driver for storage.
 
 The easiest way to run a registry as a pull through cache is to run the official
 Registry image.
+At least, you need to specify `proxy.remoteurl` within `/etc/docker/registry/config.yml`
+as described in the following subsection.
 
 Multiple registry caches can be deployed over the same back-end. A single
 registry cache ensures that concurrent requests do not pull duplicate data,