Improve readme with Testing instruction

Summary:
Currently, it is not clear how to run codegen tests in the OSS.

## Changelog
[General][Added] - Add instructions about how to add tests in the OSS

Reviewed By: cortinico

Differential Revision: D40020907

fbshipit-source-id: 073ec8bf08d2a9e8b26287763dd881371ecf561d

Author: Riccardo Cipolleschi

packages/react-native-codegen/README.md

@@ -12,3 +12,10 @@ yarn add --dev react-native-codegen
 
 [version-badge]: https://img.shields.io/npm/v/react-native-codegen?style=flat-square
 [package]: https://www.npmjs.com/package/react-native-codegen
+
+## Testing
+
+To run the tests in this package, run the following commands from the react Native root folder:
+
+1. `yarn` to install the dependencies. You just need to run this once
+2. `yarn jest react-native-codegen`.