Twitter card meta tags

Fixes #1527.

views/index.erb

@@ -10,6 +10,11 @@
   <meta property="og:type" content="website">
   <meta property="og:url" content="<%= canonical_origin %>">
   <meta property="og:image" content="/images/icon-320.png">
+  <meta property="twitter:title" content="DevDocs API Documentation">
+  <meta property="twitter:description" content="Fast, offline, and free documentation browser for developers. Search 100+ docs in one web app: HTML, CSS, JavaScript, PHP, Ruby, Python, Go, C, C++…">
+  <meta property="twitter:card" content="summary_large_image">
+  <meta property="twitter:url" content="<%= canonical_origin %>">
+  <meta property="twitter:image" content="/images/icon-320.png">
   <meta name="apple-mobile-web-app-title" content="DevDocs">
   <meta name="apple-mobile-web-app-capable" content="yes">
   <meta name="apple-mobile-web-app-status-bar-style" content="black">