demo(checkbox): add color example for checkbox (#6336)

Author: tinayuangao

src/demo-app/a11y/checkbox/checkbox-a11y.html

@@ -33,3 +33,10 @@ <h2>Nested checklist</h2>
       </div>
     </div>
 </section>
+
+<section>
+  <h2>Colored checkboxes</h2>
+  <md-checkbox color="primary">Primary</md-checkbox>
+  <md-checkbox color="accent">Accent</md-checkbox>
+  <md-checkbox color="warn">Warn</md-checkbox>
+</section>