drop some helpful text into the window

coro/http/demo/websocket/field/field.html

@@ -7,5 +7,7 @@
 <canvas id="canvas" width="1024" height="1024"></canvas>
 <div id="msgs"></div>
 <script src="field.js"></script>
+<p>Grab the field background with your mouse and move it!  Also, try
+the keys W, A, S, D, R.</p>
 </body>
 </html>