code: public void addFormField(FormField formField) throws HelloSignException {
          if (!has(DOCUMENT_FORM_FIELDS)) {
              clearList(DOCUMENT_FORM_FIELDS);
          }
          addToList(DOCUMENT_FORM_FIELDS, formField);
      }
docs: Adds the form field to this document.
  
  @param formField FormField
  @throws HelloSignException thrown if there is a problem adding the
  FormFields to this Document.