Add the example for multiple formats for issue 21 (#96)

index.html

@@ -167,7 +167,21 @@ <h3>
  <h3>
  Supporting multiple formats
  </h3>
- <aside class="issue" data-number="21"></aside>
+ <aside class="example">
+ <p>
+ The following example shows a list of potential source formats
+ that can be selected from:
+ </p>
+ <pre class="html">
+ &lt;model&gt; 
+ &lt;source src="3d-assets/teapot.usdz" type="model/vnd.pixar.usd"&gt;
+ &lt;source src="3d-assets/teapot.gltf" type="model/gltf-binary"&gt;
+ &lt;/model&gt;
+ </pre>
+ `&lt;model&gt;` source selection follows `&lt;video&gt;` selection,
+ where the top-level `src` attribute takes precedence if present, 
+ followed by the first compatible `&lt;source&gt;` element encountered.
+ </aside>
  <h3>
  Providing fallback content for legacy user agents
  </h3>