Summary:I updated the Network documentation to not lead users to use Parse, since its service is shutting down.
Closes https://github.com/facebook/react-native/pull/6140
Differential Revision: D2974665
fb-gh-sync-id: 2a784ad00f742f9f2e2ce369f8b3e1eaab0e2ad4
shipit-source-id: 2a784ad00f742f9f2e2ce369f8b3e1eaab0e2ad4