Add submit to horizontal form

docs/documentation/elements/form.html

@@ -817,6 +817,21 @@ doc-subtab: form
     </div>
   </div>
 </div>
+
+<div class="field is-horizontal">
+  <div class="field-label">
+    <!-- Left empty for spacing -->
+  </div>
+  <div class="field-body">
+    <div class="field">
+      <div class="control">
+        <button class="button is-primary">
+          Send message
+        </button>
+      </div>
+    </div>
+  </div>
+</div>
 {% endcapture %}
 {{horizontal_form_example}}
 {% highlight html %}