add responsive embed to customizer

docs/customize.html

@@ -164,6 +164,12 @@ lead: Customize Bootstrap's components, Less variables, and jQuery plugins to ge
             Panels
           </label>
         </div>
+        <div class="checkbox">
+          <label>
+            <input type="checkbox" checked value="responsive-embed.less">
+            Responsive embed
+          </label>
+        </div>
         <div class="checkbox">
           <label>
             <input type="checkbox" checked value="wells.less">