Added link to protocol discussion.

unsupported/hp-scanjet-4570c.html

@@ -104,6 +104,11 @@ U12
  http://www.microchip.com/download/lit/pline/memory/ic/21710a.pdf
     </pre>
     <h2>Comments</h2>
+    <p>
+      Some discussion about the protocol can be found in the <a
+      href="http://lists.alioth.debian.org/pipermail/sane-devel/2005-September/014618.html">sane-devel
+      mailinglist archive</a>.
+    </p>
     <p>
       Portrait scanner with TA, similar to <a
       href="hp-scanjet-4500c.html">4500C</a>. Same as 5550C, but without