Update README.md

[Stiveknx]

I took me a lot of time to figure this out, should be documented

[sageknives]

I figured it out. Basically you need to add the field data with whatever you want to get passed back

window.geofence.addOrUpdate({
id: "69ca1b88-6fbe-4e80-a4d4-ff4d3748acdb",
latitude: 50.2980049,
longitude: 18.6593152,
radius: 3000,
transitionType: TransitionType.ENTER,
notification: {
id: 1,
title: "Welcome in Gliwice",
text: "You just arrived to Gliwice city center.",
openAppOnClick: true,
data: "this is what is sent back" //this
}
}).then(function () {
console.log('Geofence successfully added');
}, function (reason) {
console.log('Adding geofence failed', reason);
})

window.geofence.onNotificationClicked = function(notificationData){
console.log(notificationData); //notificationData = "this is what is sent back"
}