Update documentation with links to Sparkfun's blog

gui/index.html

@@ -482,8 +482,8 @@ span.mainfunction {color: #993300; font-weight: bolder}
 	<p>This object has no functions to call from the Arduino sketch.  It
 		simply streams data from the I2S hardware to its 4 output ports.</p>
 	<h3>Hardware</h3>
-	<p>TODO: add detailed documentation about connecting 2 audio shields,
-		including the wire needed from pin 30 to the 2nd audio shield</p>
+	<p>See this Sparkfun blog for <a href="https://www.sparkfun.com/news/2055" target="_blank">how
+		to connect two audio adaptors for 4 channel audio</a>.
 	<p>The I2S signals are used in "master" mode, where Teensy creates
 		all 3 clock signals and controls all data timing.</p>
 	<table class=doc align=center cellpadding=3>
@@ -680,7 +680,9 @@ span.mainfunction {color: #993300; font-weight: bolder}
 	<p>This object has no functions to call from the Arduino sketch.  It
 		simply streams data from its 4 input ports to the I2S hardware.</p>
 	<h3>Hardware</h3>
-	<p>TODO: <a href="https://forum.pjrc.com/threads/29373-Bit-bang-multiple-I2S-inputs-simultaneously?p=79606#post79606" target="_blank">details</a> for how to connect 2 Teensy audio shields</p>
+	<p>See this Sparkfun blog for <a href="https://www.sparkfun.com/news/2055" target="_blank">how
+		to connect two audio adaptors for 4 channel audio</a>.  More
+		<a href="https://forum.pjrc.com/threads/29373-Bit-bang-multiple-I2S-inputs-simultaneously?p=79606#post79606" target="_blank">details</a> are also available.
 	<p>The I2S signals are used in "master" mode, where Teensy creates
 		all 3 clock signals and controls all data timing.</p>
 	<table class=doc align=center cellpadding=3>