Compare and contrast with expo-random

[pcowgill]

Compare and contrast with expo-random in the README

https://github.com/expo/expo/tree/master/packages/expo-random

[mvayngrib]

@pcowgill i don't have time atm to look into this, but feel free to submit a PR for an Alternative Approaches section in the README

[kyle-johnson]

It doesn't require react-native-unimodules! 🎉