Add libraries, picocalc notes.

libraries/arduino_picocalc_kbd/examples/GPIOExpander/GPIOExpander.ino

@@ -0,0 +1,31 @@
+#include <PCKeyboard.h>
+
+#define CARD_DET 1
+
+PCKeyboard keyboard;
+int prevValue = LOW;
+
+void setup()
+{
+  Serial.begin(9600);
+
+  Wire.setSDA(4);
+  Wire.setSCL(5);
+  Wire.begin();
+  Wire.setClock(10000);
+
+  keyboard.begin();
+  keyboard.pinMode(CARD_DET, INPUT);
+  keyboard.digitalWrite(CARD_DET, HIGH); // Enable pull-up
+}
+
+void loop()
+{
+  const int value = keyboard.digitalRead(CARD_DET);
+  if (value == prevValue)
+    return;
+
+  Serial.printf("Card Detect: %d \r\n", value);
+
+  prevValue = value;
+}