Summary:
This PR restores some [additional detail](http://facebook.github.io/react-native/releases/0.28/docs/getting-started.html) that was removed from the Android Getting Started this summer.
I'm not fully restoring the original list of steps as the React Native website should not be the source of truth for setting up an Android development environment.
Closes https://github.com/facebook/react-native/pull/9867
Differential Revision: D3887834
Pulled By: fredemmott
fbshipit-source-id: 8e3599f8945ba68f31dc9b0f79c2db7e525e7f45