Add a link in the Discover section to the Component Kitchen component catalog

A new site called [Component Kitchen](http://component.kitchen) is now live. (We'll be effectively launching it at the Polymer SHTML5 event this week.) I thought the site would make a nice addition to the Discover section.

(BTW: Please check out the site and let us know what you think! It's fine for the link to go live on webcomponents.org now, although we won't be promoting it publicly until the evening of Wednesday, April 30.)