title: Add empty document to collection
body: <p>I am trying to add a new document and collection to my database. I know I can just add a document and if the collection where it is supposed to be will be added if doe not exist. 
  My question is: how can I add an empty document to a new collection in my database?</p>
  
  <p>I have tried this code, from the android developers site:</p>
  
  <pre><code>ApiFuture&lt;WriteResult&gt; future = db.collection("cities").document("LA");
  // ...
  // future.get() blocks on response
  System.out.println("Update time : " + future.get().getUpdateTime());
  </code></pre>
  
  <p>But it gives me error in the ApiFuture part that I can't resolve.
  So I tried this:</p>
  
  <pre><code>db.collection(mail).document("tarefa");
  </code></pre>
  
  <p>(mail is a string)
  But it does not work.
  Does anyone know what I should do?</p>