title: Firestore: difference between set() and add()
body: <p>What is the difference between <code>set()</code> and <code>add()</code> in Firestore?</p>
  
  <p>I use the <code>set()</code> to add documents to my collection. But I am unable to use <code>add()</code> or understand <code>add()</code> from docs.</p>