There has been some questions as to how to actually handle this shield in the code, so I'm providing some simple examples:

MicroPython

from machine import I2C, Pin
import time
i2c = I2C(sda=Pin(4), scl=Pin(5))
while True:
    print(hex(i2c.readfrom(0x20, 1)[0]))
    time.sleep(1)

 NodeMCU

i2c.setup(0, 1, 2, i2c.SLOW)
tmr.alarm(0, 1000, 1, function ()
    i2c.start(0)
    i2c.address(0, 30, i2c.RECEIVER)
    print(string.toHex(i2c.read(0, 1)))
    i2c.stop(0)
end)

 Arduino

#include <Wire.h>
void setup() {
    Wire.begin();
    Serial.begin();
}
void loop() {
    Wire.requestFrom(0x20, 1);
    Serial.print(Wire.read(), HEX);
    delay(1000);
}

 All of the above examples scan the buttons once a second and print their state as a hexadecimal number on the serial console.