title: Lagom service reply on topic
body: <p>I'm implementing a set of Lagom services and I have one that's planned to purely read from a topic and replying back to that topic. </p>
  
  <p>I can't seem to find any docs on how to do this? All of the subscriber examples map from a message to Done and all of the publish examples are mapping from the event store events to publish external messages. </p>