Add documentation for img.is-rounded

docs/documentation/elements/image.html

@@ -28,6 +28,12 @@ meta:
 </figure>
 {% endcapture %}
 
+{% capture rounded_image %}
+<figure class="image is-128x128">
+  <img class="is-rounded" src="{{site.url}}/images/placeholders/128x128.png">
+</figure>
+{% endcapture %}
+
 {% capture retina_image %}
 <figure class="image is-128x128">
   <img src="{{site.url}}/images/placeholders/256x256.png">
@@ -58,6 +64,14 @@ meta:
   </tbody>
 </table>
 
+{% include elements/anchor.html name="Rounded images" %}
+
+<div class="content">
+  <p>You can also make rounded images, using <code>.is-rounded</code> class:</p>
+</div>
+
+{% include elements/snippet.html content=rounded_image %}
+
 {% include elements/anchor.html name="Retina images" %}
 
 <div class="content">