Mention BoostBook

[SVN r21070]

tools/index.html

@@ -43,7 +43,12 @@ platform, or for use with their own applications.</p>
   testing system reporting sources.<br>
 &nbsp;</li>
   <li><a href="inspect/index.html">Inspect </a>- The inspection tool used to 
-  detect errors in the Boost directory hierarchy.</li>
+  detect errors in the Boost directory hierarchy.<br>&nbsp;</li>
+  <li><a href="../doc/html/boostbook.html">BoostBook</a> - A Boost
+  documentation system, based on <a
+  href="http://www.docbook.org/">DocBook</a> and the <a
+  href="http://www.w3.org/Style/XSL/">Extensible Stylesheet Language
+  (XSL)</a>, used by some Boost libraries.</li>
 </ul>
 <hr>
 <p>Revised