Summary:
After pull request #12755 it is no longer necessary to manually start the packager on Windows using `react-native start`. I've updated the documentation to reflect this change, and match the Mac OS - Android instructions (same commands used to launch a project now).
Closes https://github.com/facebook/react-native/pull/13186
Differential Revision: D4790827
Pulled By: hramos
fbshipit-source-id: 5fe5f7ed20ac4b3ee537c95a9b6d7b91fd578c41