Add AudioEffectMidSide documentation & example (thanks Hedde Bosman)

Audio.h

 #include "effect_multiply.h"
 #include "effect_delay.h"
 #include "effect_delay_ext.h"
+#include "effect_midside.h"
 #include "filter_biquad.h"
 #include "filter_fir.h"
 #include "filter_variable.h"

examples/Effects/Mid_Side/Mid_Side.ino

+#include <Audio.h>
+#include <Wire.h>
+#include <SPI.h>
+#include <SD.h>
+#include <SerialFlash.h>
+
+// GUItool: begin automatically generated code
+AudioInputI2S            i2s1;           //xy=109,157
+AudioEffectMidSide       ms_enc1;        //xy=243,157
+AudioFilterBiquad        biquad1;        //xy=265,246
+AudioMixer4              mixer1;         //xy=355,73
+AudioMixer4              mixer2;         //xy=400,268
+AudioEffectMidSide       ms_dec1;        //xy=487,154
+AudioOutputI2S           i2s2;           //xy=637,154
+AudioConnection          patchCord1(i2s1, 0, ms_enc1, 0);
+AudioConnection          patchCord2(i2s1, 1, ms_enc1, 1);
+AudioConnection          patchCord3(ms_enc1, 0, mixer1, 0);
+AudioConnection          patchCord4(ms_enc1, 1, biquad1, 0);
+AudioConnection          patchCord5(biquad1, 0, mixer2, 0);
+AudioConnection          patchCord6(mixer1, 0, ms_dec1, 0);
+AudioConnection          patchCord7(mixer2, 0, ms_dec1, 1);
+AudioConnection          patchCord8(ms_dec1, 0, i2s2, 0);
+AudioConnection          patchCord9(ms_dec1, 1, i2s2, 1);
+AudioControlSGTL5000     sgtl5000_1;     //xy=119,90
+// GUItool: end automatically generated code
+
+
+const int myInput = AUDIO_INPUT_LINEIN;
+// const int myInput = AUDIO_INPUT_MIC;
+
+void setup() {
+  // The stereo line input is encoded into mid and side components.
+  // The mid component will be attenuated (mixer1), which leaves some 
+  // headroom for the side component to be increased in volume (mixer2).
+  // Furthermore, the side component is high-passed (biquad1).
+  AudioMemory(6);
+  sgtl5000_1.enable();
+  sgtl5000_1.inputSelect(myInput);
+  sgtl5000_1.volume(1.0); // output volume
+
+  ms_enc1.encode();
+  ms_dec1.decode();
+
+  // We attenuate the MID channel a little to prevent saturation when increasing the SIDE channel gain
+  mixer1.gain(0, 0.9);
+  // We increase the gain of the SIDE channel to increase stereo width
+  mixer2.gain(0, 2.0);
+  // But, we remove low frequencies from the side channel. Better for sub and doesn't get your ears twinkling with out-of-phase basses using a headphone
+  biquad1.setHighpass(0, 200, 0.7);
+  
+  Serial.begin(9600);
+  while (!Serial) ;
+  delay(3000);
+  Serial.println("Initialized mid-side example");
+}
+
+void loop() {
+}
+

gui/index.html

 		{"type":"AudioEffectDelay","data":{"defaults":{"name":{"value":"new"}},"shortName":"delay","inputs":1,"outputs":8,"category":"effect-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioEffectDelayExternal","data":{"defaults":{"name":{"value":"new"}},"shortName":"delayExt","inputs":1,"outputs":8,"category":"effect-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioEffectBitcrusher","data":{"shortName":"bitcrusher","inputs":1,"outputs":1,"category":"effect-function","color":"#E6E0F8","icon":"arrow-in.png"}},
+		{"type":"AudioEffectMidSide","data":{"shortName":"midside","inputs":2,"outputs":2,"category":"effect-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioFilterBiquad","data":{"defaults":{"name":{"value":"new"}},"shortName":"biquad","inputs":1,"outputs":1,"category":"filter-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioFilterFIR","data":{"defaults":{"name":{"value":"new"}},"shortName":"fir","inputs":1,"outputs":1,"category":"filter-function","color":"#E6E0F8","icon":"arrow-in.png"}},
 		{"type":"AudioFilterStateVariable","data":{"defaults":{"name":{"value":"new"}},"shortName":"filter","inputs":2,"outputs":3,"category":"filter-function","color":"#E6E0F8","icon":"arrow-in.png"}},
         <tr class=odd><td align=center>In 0</td><td>Signal Input</td></tr>
         <tr class=odd><td align=center>Out 0</td><td>Signal Output</td></tr>
     </table>
-    <h3>Parameters</h3>
+    <h3>Functions</h3>
     <p class=func><span class=keyword>bits</span>(xcrushBits);</p>
     <p class=desc>xcrushBits sets the bitdepth, from 1 to 16. A Value of 16
     does not crush the bitdepth, and is effectively a passthru for this part
     </div>
 </script>
 
+<script type="text/x-red" data-help-name="AudioEffectMidSide">
+	<h3>Summary</h3>
+	<p>Convert stereo signals to/from Mid-Side format.
+		Mid-Side encoding can be used to increase stereo width, make the lower
+		frequencies mono (to please your sub), or as the basis of audio compression.</p>
+	<h3>Audio Connections</h3>
+	<table class=doc align=center cellpadding=3>
+		<tr class=top><th>Port</th><th>While<br>Encoding</th><th>While<br>Decoding</th></tr>
+		<tr class=odd><td align=center>In 0</td><td>Left Input</td><td>Mid Output</td></tr>
+		<tr class=odd><td align=center>In 1</td><td>Right Input</td><td>Side Output</td></tr>
+		<tr class=odd><td align=center>Out 0</td><td>Mid Input</td><td>Left Output</td></tr>
+		<tr class=odd><td align=center>Out 1</td><td>Side Input</td><td>Right Output</td></tr>
+	</table>
+	<h3>Functions</h3>
+	<p class=func><span class=keyword>encode</span>();</p>
+	<p class=desc>Configure this object to encode from stereo to Mid-Side format.</p>
+	<p class=func><span class=keyword>decode</span>();</p>
+	<p class=desc>Configure this object to decode from Mid-Side format back to stereo signals.</p>
+
+	<h3>Examples</h3>
+	<p class=exam>File &gt; Examples &gt; Audio &gt; Effects &gt; Mid_Side</p>
+	<h3>Notes</h3>
+	<p>Many interesting stereo effects can be achieved by manipulating Mid-Side signals.</p>
+	<p>Normally a pair of these objects are used, one to encode, then additional
+		gain/attenuation or effects applied to the Mid-Side signals, and finally
+		decoding back to stereo signals</p>
+	<p>To prevent saturation, halving is done in the encoding, that is:</p>
+	<p>Mid = (left+right)/2</p>
+	<p>Side = (left-right)/2</p>
+	<p>And to decode:</p>
+	<p>Left = Mid+Side</p>
+	<p>Right = Mid-Side</p>
+</script>
+<script type="text/x-red" data-template-name="AudioEffectMidSide">
+    <div class="form-row">
+        <label for="node-input-name"><i class="fa fa-tag"></i> Name</label>
+        <input type="text" id="node-input-name" placeholder="Name">
+    </div>
+</script>
+
 <script type="text/x-red" data-help-name="AudioFilterBiquad">
 	<h3>Summary</h3>
 	<div>