add local file warning

In Javascript/airgap.html

@@ -18,7 +18,7 @@
 		<input type="button" value="Play Song" onclick="start()">
 		<input type="button" value="Stop Song" onclick="end()"></br></br>
 		<textarea id="logs" style="width:70%;min-height:100px">Tested with Chrome at 1560Khz</textarea>
-		<p style="font-size:14px">Feel free to edit the code below or copy and paste any <em>valid</em> code.<br>Column one is time in <i>milliseconds</i>, and column two is <i>frequency</i>.</p>
+		<p style="font-size:14px">Feel free to edit the code below or copy and paste any <em>valid</em> code.<br>Column one is time in <i>milliseconds</i>, and column two is <i>frequency</i>.<br>If you are opening this file locally (file://) Chrome will give you errors. Try using <code>php -S localhost:8000</code> or similar for a quick web server.</p>
 		<textarea id="tones" style="width:70%;min-height:200px">
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