Documentation improvement request

[yannbf]

Hey there 👋 I was recently setting up a project to use Storybook for RN and went through a few bumps. I feel like a few changes to the documentation (README file) could go a long way:

1 - I'd like to understand what Storybook does in my app. Seems like it replaces the app, but it wasn't clear for me at first.
2 - There's some wording that mentions "the only left to do" which are a bit misleading
3 - It would be nice to restructure the docs a bit to "choose my own adventure" - whether I use metro or expo

I hope you consider these requests. Thanks!