Update README (#23779)

Summary:
Clean up and make the README look a bit nicer. Inspired by https://github.com/gatsbyjs/gatsby/blob/master/README.md.

This PR brings back an introduction to React Native that focuses on our strengths. I'm happy to punt updates to the marketing copy for later, if we want to focus on updating the README layout first.

![readme](https://user-images.githubusercontent.com/165856/53846592-59f3b700-3f62-11e9-95cf-4391c74460f4.png)

[General][Changed] - Nicer README.
Pull Request resolved: https://github.com/facebook/react-native/pull/23779

Differential Revision: D14341562

Pulled By: hramos

fbshipit-source-id: 87dba4fe4e3598f638fdc499347a2c9aedde8f8d