Added example for user defined callback

An additional example sampleValuesWithCallback is added which shows how to use user defined callbacks to distinguish between the source of the peek() values

arm/libraries/I2S/examples/sampleValuesWithCallback/sampleValuesWithCallback.ino

@@ -0,0 +1,61 @@
+#include <I2S.h>
+
+/*
+ * This example reads audio data from Infineon's IM69D130 Microphone Shield2Go 
+ * and prints it on the serial monitor - additionally, it uses a custom callback
+ * to show how to distinguish between the peeked values for LOW and HIGH word select.
+ * 
+ * Open the serial monitor with baudrate of 1000000 to see the results.
+ * 
+ * Note: as 2 channels are used, the values returned from I2S.read() will be from both channels and thus look noisy
+ * In this case, both values need to be separated.
+ */
+
+volatile int32_t lastValueLow = 0;
+volatile int32_t lastValueHigh = 0; 
+
+void customCallback(){
+  if(I2S.getSampleInformation() == NO_MICROPHONE) return;
+
+  if(I2S.getSampleInformation() == MICROPHONE_LOW){
+    lastValueLow = I2S.peek();
+  }
+  if(I2S.getSampleInformation() == MICROPHONE_HIGH){
+    lastValueHigh = I2S.peek();
+  }
+}
+
+void setup()
+{
+  Serial.begin(1000000);
+  Serial.println("Begin of I2S microphone");
+  // Disable all microphones
+  I2S.disableMicrophones();
+  // Enable the microphone in stereo mode
+  I2S.enableMicrophoneLow();
+  I2S.enableMicrophoneHigh();
+
+  // Register the custom callback function
+  I2S.onReceive(customCallback);
+
+  // Start I2S with I2S_PHILIPS_MODE, 11 kHz sampling rate and 20 bits per sample
+  // Returns 0 if everything okay, otherwise value > 0
+  I2S.begin(I2S_PHILIPS_MODE, 11000, 20);
+}
+
+void loop()
+{
+  // I2S is very demanding in terms of data rate
+  // Uncomment the line with I2S.getOverflow() to see whether the internal buffer has an overflow or not
+  while (I2S.available() > 0)
+  {
+    // Read one value from the internal buffer and return it on the serial console
+    Serial.println(I2S.read());
+    // if(I2S.getOverflow() == true) Serial.println("Overflow");
+  }
+  // Print the peeked values from the defined callback
+  Serial.println("Last peeked value for LOW");
+  Serial.println(lastValueLow);
+  Serial.println("Last peeked value for HIGH");
+  Serial.println(lastValueHigh);
+}