Added twitter meta tags.

index.html

@@ -4,6 +4,8 @@
 		<meta charset="utf-8">
 		<title>three.js – JavaScript 3D library</title>
 		<meta name="viewport" content="width=device-width, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0">
+		<meta name="twitter:card" content="summary">
+		<meta name="twitter:site" content="@threejs_org">
 		<meta property="og:image" content="files/share.png">
 		<link rel="shortcut icon" href="files/favicon.ico">
 		<link rel="stylesheet" type="text/css" href="files/main.css">