code: public AddAction append(ByteBuffer... values) {
          return new AddAction(this, new LiteralOperand(
                  new LinkedHashSet<ByteBuffer>(Arrays.asList(values))));
      }
docs: Returns an <code>AddAction</code> for build expression that would append
  the specified values to this binary 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.