Add data-menu-button-contents example

Author: dgraham

README.md

@@ -25,7 +25,7 @@ import '@github/details-menu-element'
 </details>
 ```
 
-Use `data-menu-button` and `data-menu-button-text` to have button text updated on menu item activation.
+Use `data-menu-button` and `data-menu-button-text` to update the button's text on menu item activation.
 
 ```html
 <details>
@@ -38,6 +38,19 @@ Use `data-menu-button` and `data-menu-button-text` to have button text updated o
 </details>
 ```
 
+Use `data-menu-button` and `data-menu-button-contents` to update the button's HTML content on menu item activation.
+
+```html
+<details>
+  <summary>Preferred robot: <span data-menu-button>None</span></summary>
+  <details-menu role="menu">
+    <button type="button" role="menuitem" data-menu-button-contents><img src="hubot.png"> Hubot</button>
+    <button type="button" role="menuitem" data-menu-button-contents><img src="bender.png"> Bender</button>
+    <button type="button" role="menuitem" data-menu-button-contents><img src="bb8.png"> BB-8</button>
+  </details-menu>
+</details>
+```
+
 Use `label[tabindex="0"][role=menuitemradio/menuitemcheckbox]` when dealing with radio and checkbox inputs menu items. Check states of the input element and the label will be synchronized.
 
 ```html