code: public ApiFuture<DocumentReference> add(Object pojo) {
      Object converted = CustomClassMapper.convertToPlainJavaTypes(pojo);
      if (!(converted instanceof Map)) {
        FirestoreException.invalidState("Can't set a document's data to an array or primitive");
      }
      return add((Map<String, Object>) converted);
    }
docs: Adds a new document to this collection with the specified POJO as contents, assigning it a
  document ID automatically.
  
  @param pojo The POJO that will be used to populate the contents of the document
  @return An ApiFuture that will be resolved with the DocumentReference of the newly created
  document.
  @see #document()