Add CodeMirror 6 banner to front page

Author: marijnh

index.html

@@ -18,10 +18,13 @@
   .CodeMirror { height: auto; border: 1px solid #ddd; }
   .CodeMirror-scroll { max-height: 200px; }
   .CodeMirror pre { padding-left: 7px; line-height: 1.25; }
-  .banner { background: #ffc; padding: 6px; border-bottom: 2px solid silver;  }
-  .banner div { margin: 0 auto; max-width: 700px; text-align: center; }
+  .banner { background: #ffc; padding: 6px; border-bottom: 2px solid silver; text-align: center }
 </style>
 
+<div class=banner>
+  ‼️ Check out the upcoming <a href="https://codemirror.net/6/">CodeMirror 6</a>, a much-improved rewrite! ✨
+</div>
+
 <div id=nav>
   <a href="https://codemirror.net"><h1>CodeMirror</h1><img id=logo src="doc/logo.png"></a>