Adds task link to index/subnav[#134606323]

Author: sleslie

index.html.md.erb

@@ -40,6 +40,8 @@ ___
 
 <li><a href="./deploy-apps/cf-scale.html" class="subnav">Scaling an Application Using cf scale</a></li>
 
+<li><a href="./using-tasks.html" class="subnav">Using Tasks</a></li>
+
 <%=vars.autoscaler_bullet%>
 
 <li><a href="./deploy-apps/environment-variable.html" class="subnav">Cloud Foundry Environment Variables</a></li>