Add a banner pointing at the version 6 announcement

Author: marijnh

index.html

@@ -18,8 +18,14 @@
   .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; }
 </style>
 
+<div class=banner><div>
+  CodeMirror is evolving! Check out our <a href="https://codemirror.net/6/">announcement</a> about the upcoming redesigned, accessible, mobile-friendly version.
+</div></div>
+
 <div id=nav>
   <a href="https://codemirror.net"><h1>CodeMirror</h1><img id=logo src="doc/logo.png"></a>