I don't have an RSS feed for this, and I don't yet have a simple way to provide it since the stats are generated dynamically; there's no trigger to automatically update it for only when there's a change. But, if you want to include just the rankings table on your site, you can add an iframe with this URL for the src:
http://lol.shrapnelgames.com/ViewRatings.asp?inset=yes