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