added missing make_torrent documentation

docs/python_binding.html

@@ -85,6 +85,9 @@ python setup.py install
 </pre>
 </li>
 </ol>
+<p>This requires that you have built and install libtorrent first. The setup.py relies
+on pkg-config to be installed as well. It invokes pkg-config in order to link
+against libtorrent.</p>
 </div>
 <div class="section" id="building-using-boost-build">
 <h2>building using boost build</h2>