Add help text to demo page

demo.html

@@ -10,7 +10,8 @@
 		<div class="debug-data">
 			<ul>
 				<li>Points: <output id="output-point-count"></output> &#10142; <output id="output-simple-point-count"></output></li>
-				<li>Min Area: <input type="range" id="input-epsilon" value="2" min="0" max="100" step="0.05" /> <output id="output-epsilon"></output>
+				<li>Min Area: <input type="range" id="input-epsilon" value="2" min="0" max="100" step="0.05" /> <output id="output-epsilon"></output></li>
+				<li><small><em>Click and drag to draw a line. <kbd>1</kbd>: Toggle original line (blue), <kbd>2</kbd>: Toggle simplified line (green)</li>
 			</ul>
 		</div>