Fix ProGuard/R8 crash with React Native 0.87 and AGP 9

[AlexCernik]

When using React Native 0.87 with AGP 9 and R8/ProGuard enabled, the app can crash on startup with the following error:

Wrong configuration of external resource loader:
java.lang.AssertionError:
Class net.time4j.android.spi.AndroidResourceLoader has no zero argument constructor

This was not reproducible with React Native 0.86.2 and NDK 27.

This PR updates the ProGuard documentation with the rules required to prevent R8 from removing the zero-argument constructor used by Time4J:

-keep public class net.time4j.android.ApplicationStarter
-keep public class net.time4j.PrettyTime
-keep public class net.time4j.android.spi.AndroidResourceLoader {
    public <init>();
}