This adds information on how to get the library working on React Native to the readme.

To verify that this works:

1. creating a new React Native project (`npx react-native init uuidtest`)
2. added the dependencies (`npm add react-native-get-random-values uuid`)
3. imported the polyfill (`import 'react-native-get-random-values'` in `index.js`)
4. and displayed an UUID in the app (`import * as uuid from 'uuid'` and `{uuid.v4()}` in `App.js`)

Closes #375

* link to React Native readme section from rng error
* For linkability, move react-native docs to separate section.
* Link to readme in src/rng-browser.js

Co-Authored-By: Robert Kieffer <robert@broofa.com>

It doesn't make sense to explicitly use window. only once in the chain.