Add a link to the new acks.html file.

Doc/html/about.html

@@ -45,6 +45,8 @@
       and PostScript for printing, from the <a
         href="http://www.python.org/">Python Web site</a>.
 
+    <p>A <a href="acks.html">list of contributors</a> is available.
+
     <h2>Comments and Questions</h2>
 
     <p> General comments and questions regarding this document should