Clarify project's support status in readme
Clarify project's support status in readme

Jams a fresh new "Support" section at the top of the readme that tries to kindly outline this project's support status.

Fixes #191.

/cc @bsudekum @tmcw
bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl,bsudekum/react-native-mapbox-gl