title: Firestore add security rule to allow reads only from mobile app
body: <p>I am making a mobile app with Firestore and React Native.</p>
  <p>There is no authentication system and upon starting the app, the user download a collection from Firestore.</p>
  <pre><code>rules_version = '2';
  service cloud.firestore {
    match /databases/{database}/documents {
      match /{document=**} {
         allow read: if true;
      }
    }
  }
  </code></pre>
  <p>As of right now, the rule allows everyone to read from the database. Is it possible/advisable to limit the traffic to only our mobile app?</p>