add link to Piazza

Author: jhellerstein

index.html

@@ -60,6 +60,7 @@ <H2>Resources</H2>
 <ul>
   <li><a href="https://github.com/programthecloud/ptcrepo/wiki/Announcements">Announcements</a></li>
   <li><a href="https://github.com/programthecloud/ptcrepo/tree/gh-pages/lectures">Lecture notes</a></li>
+  <li><a href="https://piazza.com/class#spring2013/cs19417">Piazza discussion group</a></li>
   <li><a href="https://github.com/programthecloud/ptcrepo/wiki/Optional-Reading">Optional Readings</a></li>
   <li><a href="https://github.com/programthecloud/ptcrepo/wiki/FAQ">FAQ</a></li>
 </ul>