code: @Nonnull
    public ApiFuture<WriteResult> create(@Nonnull Map<String, Object> fields) {
      WriteBatch writeBatch = firestore.batch();
      return extractFirst(writeBatch.create(this, fields).commit());
    }
docs: Creates a new Document at the DocumentReference's Location. It fails the write if the document
  exists.
  
  @param fields A map of the fields and values for the document.
  @return An ApiFuture that will be resolved when the write finishes.