security: link it

Author: Edward Thomson

_layouts/default.html

@@ -25,6 +25,7 @@
             <li><a href="https://github.com/libgit2/libgit2/releases">Downloads</a></li>
             <li><a href="http://libgit2.github.com/libgit2/">Reference</a></li>
             <li><a href="/docs">Learning</a></li>
+            <li><a href="/security">Security</a></li>
             <li><a href="https://github.com/libgit2/libgit2">Source</a></li>
           </ul>
         </nav>