Document the Analytics Sample App

[hiranya911]

It's not clear what this app is supposed to do, and what the user is supposed to see. Some explanation in the README would be very useful.

[samtstern]

@hiranya911 that's a good point. Also with the recent release of Firebase Analytics DebugView we can provide a real-time view of the events logged by this app.

I will update the README to reflect this.