code: public void addDocument(Document doc, int order) throws HelloSignException {
          if (doc == null) {
              throw new HelloSignException("Document cannot be null");
          }
          try {
              documents.add(order, doc);
          } catch (Exception ex) {
              throw new HelloSignException(ex);
          }
      }
docs: Adds a Document to the signature request at the specific order.
  
  @param doc Document
  @param order int
  @throws HelloSignException thrown if null is provided or there is a
  problem attaching the Document.