Add meta description to index.html (#124)

index.html

@@ -24,7 +24,9 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta name="referrer" content="no-referrer">
+
     <title>Threema Web</title>
+    <meta name="description" content="Chat from your desktop with Threema Web and have full access to all chats, contacts and media files.">
 
     <!-- Favicon -->
     <link rel="icon" href="img/favicon.ico?v=[[VERSION]]" type="image/x-icon"/>