Summary:
Motivation: Fix the documentation so that others don't stumble.
**Test plan**: Integrating RN into an existing Android app doesn't cause the `ConnectivityManagerCompat` exception anymore.
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The instructions where still pointing at maven central. This updates the dependency and provides instructions on how to reference the local maven folder in `node_modules`.
Closes https://github.com/facebook/react-native/pull/7586
Differential Revision: D3309932
fbshipit-source-id: 8e4fc4c0ae5641af886c9d9a0feb1c8172e94fcf