Summary:This change adds the `flow/` folder to the generated `.flowconfig` in new/upgraded projects. The absence of this folder was causing flow bugs to appear in projects consuming react-native that weren't visible in react-native itself. By including the same definition in consuming projects these errors disappear. Fixes https://github.com/facebook/react-native/issues/6428.
**Test plan (required)**
Tested `react-native upgrade` with this change and ensured that the generated `.flowconfig` works and didn't throw flow errors.
Closes https://github.com/facebook/react-native/pull/6430
Differential Revision: D3071701
fb-gh-sync-id: f28f4d8f7e63669386766b6f226144adeda32c85
shipit-source-id: f28f4d8f7e63669386766b6f226144adeda32c85