code: public AddAction append(String... values) {
          return new AddAction(this, new LiteralOperand(
                  new LinkedHashSet<String>(Arrays.asList(values))));
      }
docs: Returns an <code>AddAction</code> for building expression that would
  append the specified values to this string set; or if the attribute does
  not already exist, add the new attribute and the value(s) to the item.
  <p>
  In general, DynamoDB recommends using SET rather than ADD.