Add comment.

src/social-likes.js

@@ -34,6 +34,7 @@
 	 */
 	var services = {
 		facebook: {
+			// https://developers.facebook.com/docs/reference/fql/link_stat/
 			counterUrl: 'https://graph.facebook.com/fql?q=SELECT+total_count+FROM+link_stat+WHERE+url%3D%22{url}%22&callback=?',
 			convertNumber: function(data) {
 				return data.data[0].total_count;