Editorial: say what the draft represents

Fixes #1007.

spec.html

@@ -74,6 +74,8 @@ <h1>Contributing to this Specification</h1>
     </li>
   </ul>
   <p>Refer to the <emu-xref href="#sec-colophon">colophon</emu-xref> for more information on how this document is created.</p>
+  <h1>About this Specification</h1>
+  <p>This document at <a href=".">https://tc39.es/ecma262/</a> is the most accurate up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any <a href="https://github.com/tc39/proposals/blob/master/finished-proposals.md">finished proposals</a> (those that have reached Stage&nbsp;4 in the <a href="https://tc39.es/process-document/">proposal process</a> and thus are implemented in several implementations and will be in the next practical revision) since that snapshot was taken.</p>
 </div>
 <emu-intro id="sec-intro">
   <h1>Introduction</h1>