title: firestore - retrieve data by filtering arrayField
body: <p>I am storing data in firestore in the format</p>
  <pre><code>feed: { userID, title, description, ..., hideFor[userID] }
  </code></pre>
  <p>I want to query feeds such that loggedin user should not see the feed if his/her id is listed in <code>hideFor[]</code> array. Which firestore query method/operator can help me in this?</p>