Add instructions to test RNTester with Hermes on Android (#32085)

Summary:
Title said it all.

## Changelog

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[Internal]

Pull Request resolved: https://github.com/facebook/react-native/pull/32085

Reviewed By: TheSavior

Differential Revision: D30555218

Pulled By: Huxpro

fbshipit-source-id: 857ee81d4f402209bb2db2e1f4a2956d46ea3a54