add note to button dropdowns about js plugin dependency per #1746

docs/components.html

@@ -248,7 +248,9 @@
             <li><a href="#">Separated link</a></li>
           </ul>
         </div><!-- /btn-group -->
-      </div>
+      </div><!-- /btn-toolbar -->
+      <hr>
+      <p><span class="label label-info">Heads up!</span> Button dropdowns require the <a href="./javascript.html#dropdowns">Bootstrap dropdown plugin</a> to function.</p>
     </div>
     <div class="span8">
       <h3>Example markup</h3>