Summary:
Adds a note that the `Modal` component is only available in iOS.
I hit a bit of a pickle this weekend after realizing a handful of components I built will have to be rewritten. Unfortunately there's no notes in the documentation that the support is limited. Hopefully this can contribute towards avoiding those situations
Closes https://github.com/facebook/react-native/pull/4592
Reviewed By: svcscm
Differential Revision: D2727634
Pulled By: androidtrunkagent
fb-gh-sync-id: 2d0efcca8e17d16cf63d592e235261cea63e59ea
ca9f0adee2