Tracking Push API

[karolklp]

Hi!
I think there's lack of valid information about browsers' support of Push and Notification API. They're not part of SW spec, but so does BackgroundSync, and a lot of people intents to use service worker for notifications. There are still details like lack of support of push payload, which is present in Android Push, which people find confusing.
What do you think about including such info in this repo?

[styfle]

Is this a duplicate of #47