title: how to add sub collection to a document in firestore?
body: <p>there is no documentation about how add sub collection in a document in firestore,
  how to add a sub collection by using a web app?
  <a href="https://i.stack.imgur.com/gqtVl.png" rel="noreferrer">even tried like this but didn't worked</a>.
  how can i add a sub collection in apps by using code?
  is there any way to do it or it will be better to use firebase real time database?
  if it so, please let me know how to do it in firebase as well<a href="https://i.stack.imgur.com/OGhws.png" rel="noreferrer">i want to add like this</a>
  <a href="https://i.stack.imgur.com/JHmTE.png" rel="noreferrer">i did this</a> but iam getting an error like this <a href="https://i.stack.imgur.com/zhcGz.png" rel="noreferrer">the error says this</a></p>