add example

example.js

@@ -0,0 +1,17 @@
+'use strict';
+
+const MPR121 = require('./index.js'),
+      mpr121  = new MPR121(0x5A, 1);
+
+// listen for touch events
+mpr121.on('touch', (pin) => console.log(`pin ${pin} touched`));
+
+// listen for release events
+mpr121.on('release', (pin) => console.log(`pin ${pin} released`));
+
+// listen for changes to the state of a specific pin
+mpr121.on(3, (state) => console.log(`pin 3 is ${state ? 'touched' : 'released'}`);
+
+// check the current state of a specific pin synchronously
+const state = mpr121.isTouched(2);
+console.log(`pin 2 is ${state ? 'touched' : 'released'}`);