docs(android): add required Proguard rule for CodePush

[jackyrusly]

Description

When Proguard / R8 is enabled, react-native-code-push may fail to apply the updated JS bundle on the first restart after an update.

This happens because getReactHostDelegate uses reflection to access reactHostDelegate from ReactHostImpl:

Field field = clazz.getDeclaredField("reactHostDelegate");

If Proguard obfuscates or removes this field, the reflection lookup fails at runtime and the CodePush bundle is not applied on first restart.

[kmsbernard]

Sorry for the long wait, and thank you for opening this PR.

This points out a real issue with ProGuard. After looking into it, I think this is better handled by fixing the library’s ProGuard rules directly rather than documenting an extra setup step for users, so I opened #54 with that change.

If you think there’s anything missing in that approach, feedback is very welcome.