Add `routeStack` prop to docs

[voronianski]

I see routeStack prop in getLeftButtonElement method. So it's actually showing "Back" button even on initial route because never gets into https://github.com/Kureev/react-native-navbar/blob/master/index.js#L44

Should I keep the stack up to date manually?

[Kureev]

Nope. Yesterday I talked to @ericvicenti about arguments Navigator's renderScene takes. So, in the short future, when this PR will be merged, you'll be able to track routeStack in the renderScene(third parameter will be state which contains routeStack).

[voronianski]

👍

[Kureev]

As far as FB guys changed their mind, now it's changed to use navigator.getCurrentRoutes().
FYI @voronianski