Add link to twitter docs

app/views/layouts/_head.html.haml

 %head{prefix: "og: http://ogp.me/ns#"}
   %meta{charset: "utf-8"}
   %meta{'http-equiv' => 'X-UA-Compatible', content: 'IE=edge'}
+  %meta{name: 'referrer', content: 'origin-when-cross-origin'}
 
   %meta{name: "description", content: page_description}
-  %meta{name: 'referrer',    content: 'origin-when-cross-origin'}
 
   -# Open Graph - http://ogp.me/
   %meta{property: 'og:type',        content: "object"}
@@ -13,6 +13,7 @@
   %meta{property: 'og:image',       content: page_image}
   %meta{property: 'og:url',         content: request.base_url + request.fullpath}
 
+  -# Twitter Card - https://dev.twitter.com/cards/types/summary
   %meta{property: 'twitter:card',         content: "summary"}
   %meta{property: 'twitter:title',        content: page_title}
   %meta{property: 'twitter:description',  content: page_description}