Update README of the NowInAndroid fork for Declarative Gradle

It would be nice to update README.md in https://github.com/gradle/nowinandroid/tree/main-declarative so that it points back to Declarative Gradle and summarizes the customizations made in the repository compared to upstream. We also do not publish the app to Android Play Store, so such confusing bits could be removed too in the Declarative Gradle branch 