README update
README update

Summary: Add instructions for building and running our RNW sample application.

Reviewed By: antonk52

Differential Revision: D39305056

fbshipit-source-id: 2adb41d713381c1b79ea5fe55a4f8a40d43e7f4c

facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper,facebook/flipper