Add dim command

hardware/sensehat/sensehat.html

@@ -130,6 +130,10 @@ To scroll a single character, append a blank space after it - <code>"A "</code>.
     <li><code>msg.background</code> - the colour of the background, default: <code>off</code></li>
     <li><code>msg.speed</code> - the scroll speed. A value in the range 1 (slower) to 5 (faster), default: <code>3</code></li>
 </ul>
+
+<p><b>Set the screen brightness</b></p>
+<p>Format: <code>D&lt;level&gt;</code></p>
+<p><code>level</code> must be 0 (low) or 1 (high).</p>
 </script>
 <script type="text/javascript">
     RED.nodes.registerType('rpi-sensehat in',{