Add note about half wave rectifier effect using waveshaper

gui/index.html

 	<p class=exam>File &gt; Examples &gt; Audio &gt; Effects &gt; Vocoder19Band
 	</p>
 	<h3>Notes</h3>
-	<p>
+	<p>To create a half wave rectifier effect, use the waveshape effect.  See
+		<a href="https://forum.pjrc.com/threads/62091?p=255367&viewfull=1#post255367">this forum message</a> for a half-wave rectifier example.
+	</p>
     </p>
 </script>
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