title: Firestore publicly writable collection
body: <p>When a user is created I need to add some additional data about the user (e.g. name).</p>
  
  <p>So I need a publicly writable collection. However I've tried adding <code>create</code> and <code>update</code> (I think that's for <code>set</code> and <code>add</code> respectively), but I'm still getting  "Error: Missing or insufficient permissions." when trying to do:</p>
  
  <pre><code>db.collection('newUsers').add({
    firstName,
    lastName,
  });
  </code></pre>
  
  <p>With this rule:</p>
  
  <pre><code>service cloud.firestore {
    match /databases/{database}/documents {
      match /newUsers {
        allow create, update;
      }
    }
  }
  </code></pre>
  
  <p>How can I add an entry to that collection without being authed?</p>