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

packages/rn-tester/README.md

@@ -24,9 +24,12 @@ You'll need to have all the [prerequisites](https://github.com/facebook/react-na
 
 Start an Android emulator.
 
-    cd react-native
-    ./gradlew :packages:rn-tester:android:app:installJscDebug
-    ./scripts/packager.sh
+```sh
+cd react-native
+# In order to use Hermes engine, run `installHermesDebug` instead.
+./gradlew :packages:rn-tester:android:app:installJscDebug
+./scripts/packager.sh
+```
 
 _Note: Building for the first time can take a while._