Add a sound warning and mute guide

Page visitors may be surprised by the sounds and not know how to turn
them off. The answer is "you don't", but practically the browser tab can
be muted instead.

www/index.html

@@ -72,6 +72,11 @@
       <p>
         A (work in progress) version of the Asteroids arcade game.
       </p>
+      <p>
+        <em>Sound Warning!</em> The game now has sound effects, but there are no controls on the page for changing the
+        volume (including to mute them). You can mute the browser tab by pressing <code>ctrl</code> + <code>m</code>.
+        Proper volume controls are coming soon.
+      </p>
     </article>
     <article>
       <h3>Controls</h3>