W3C validator given warning: No DOCTYPE found! for Kohana 3 Feed Class

I am using the following code of Feed Class from Kohana 3 to create rss feed.

$this->response->headers('Content-Type', 'text/xml; charset=utf-8');  
$this->response->body(Feed::create($info, $items));

On validation with W3C as you may check on this link: http://ift.tt/fnf73e given the result stating that document was successfully checked as well-formed XML! but with the following warning:

No DOCTYPE found! Checking XML syntax only.

The DOCTYPE Declaration was not recognized or is missing. This probably means that the Formal Public Identifier contains a spelling error, or that the Declaration is not using correct syntax, or that your XML document is not using a DOCTYPE Declaration.

Please advise how to solve this warning. What kind of DOCTYPE Declaration I shall use and where to put the code in the file of class Kohana_Feed. Thanks.