Summary:
Adding documentation for the CLI for 2 reasons:
1. It's not immediately clear to most how the CLI is part of `react-native` and not the node module they installed. This begins clarifying.
2. I plan on adding some additional documentation to this section pending an upcoming PR I've discussed with Mike.
screen shot provided:
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Closes https://github.com/facebook/react-native/pull/11097
Differential Revision: D4237028
Pulled By: mkonicek
fbshipit-source-id: c1dc50fe1be7f6947a33ee6472b5862306888055