[docs] Update README with instructions for use in EAS Build

[gregfenton]

It would be great if the README gave instructions on how to use this cool library with EAS Build + Managed Workflow. As it is currently documented, it seems to lead newcomers towards using the Bare Workflow and/or the Expo "classic build".

[henninghall]

Hi, I don't have much experience with expo myself so it would be very appreciated if you could send a PR with readme improvements that clarify this!