Add intro usage text to example

Mostly for my edification, list the IO pins and where to find them on
the D1 mini and other boards, and how to use the AIY Voice Hat.

Author: earlephilhower

libraries/esp8266/examples/I2SInput/I2SInput.ino

@@ -2,6 +2,29 @@
    I2S stereo microphone (input) example
    Run using the Arduion Serial Plotter to see waveform.
    Released to the Public Domain by Earle F. Philhower, III
+
+   For the Google AIY Voice Hat Microphone daughterboard, part
+   of the Raspberry Pi AIY cardboard box, the I2S stereo pinout
+   looking at the board top with the RPI logo on the left hand
+   side:
+            +--   ------------------------------------  --+
+   left RPI | (1) GND (2) DIN (3) BCLK (4) LRCLK (5) 3.3V | AIY right
+            +---------------------------------------------+
+
+   The I2S pins are on different pins depending on your board.
+   The *internal GPIO number* which is NOT NECESSARIALY the
+   same as the pin numbers, are as follows:
+   I2SI_DATA     = GPIO12
+   IS2I_BCK      = GPIO13
+   I2SI_WS/LRCLK = GPIO14
+
+   On the D1 mini the I2SI pins map to the following D pins:
+   I2SI_DATA     = GPIO12 = D6
+   IS2I_BCK      = GPIO13 = D7
+   I2SI_WS/LRCLK = GPIO14 = D5
+
+   Expect different D pins on different ESP8266 boards, and of
+   course be sure to wire up VCC(3.3V) and GND.
 */
 
 #include <ESP8266WiFi.h>