Add help for TOC toggle

Author: jordanmccullough

_layouts/curriculum.html

@@ -23,6 +23,11 @@ <h4 class="modal-title" id="myModalLabel">Help / Shortcuts</h4>
             <td>Hide/show help</td>
           </tr>
 
+          <tr>
+            <td><code>shift</code>+<code>t</code></td>
+            <td>Hide/show table of contents</td>
+          </tr>
+
           <tr>
             <td><code>?teacher=[github-user]</code></td>
             <td>Auto-generated teacher slide</td>