added link to lecture notes on bittorrent introduction

docs/index.html

@@ -27,6 +27,10 @@
 <li><a class="reference" href="http://libtorrent-ruby.rubyforge.org/">ruby bindings</a></li>
 <li><a class="reference" href="http://code.google.com/p/python-libtorrent/">python bindings</a></li>
 </ul>
+<hr class="docutils" />
+<ul class="simple">
+<li><a class="reference" href="bittorrent.pdf">Introduction, slides</a></li>
+</ul>
 </div>
 <div id="libraryBody"><div class="section" id="libtorrent">
 <h1><a name="libtorrent">libtorrent</a></h1>