Summary:
Adds a script that uses `react-native-codegen` to generate FBReactNativeSpec.
The generated output should not be considered ready for production use at this time.
The goal of adding this script at this time is to demonstrate the current status of native modules specs code generation in open source.
For example, the generated output may be used in RNTester, with some modifications due to some naming differences in react-native-codegen's output when compared to the FBReactNativeSpec files generated by the old codegen.
Usage:
```
./scripts/generate-native-modules-specs.sh ./codegen-out
```
Changelog: [Internal]
Reviewed By: TheSavior
Differential Revision: D21471004
fbshipit-source-id: 5ff3c57807d9ba2c91dc7fe32d227d218732b059
Héctor Ramos
91a49d8827