title: Flutter Firestore update if exist if not create
body: <p>I would like to know if specific document exists, update that document and if not create that document is possible in Firestore or not.
  For example, user entered some tag with post, then look like this<br>
   <code>Firestore.instance.collection('tags').document(tag here).updateData()</code>.
  Is there any way if that document doesn't exist, then create method or do I have to check if exists before writing it?</p>