Summary:
Add a message to let people know they can use the `--simulator` flag to run their apps on different simulators instead of the default "iPhone 6"
Closes https://github.com/facebook/react-native/pull/8078
Differential Revision: D3464912
Pulled By: JoelMarcey
fbshipit-source-id: b59d5061d2b3501618602932fcc285bac99b7573
Joel Cloralt