docs: add information on usage from React Native

[LinusU]

Closes #375

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

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I've verified that this works by:

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)

[LinusU]

ping @ctavan, this PR is per my comments in #375 ☺️

[SimenB]

Does this work in Expo? If not (seems like it's a native lib), a separate section for expo would be ❤️

[geraintwhite]

After including that polyfill the errors change to the following on both iOS and Android:

'Unhandled promise rejection', { [TypeError: null is not an object (evaluating 'RNGetRandomValues.getRandomBase64')]

[LinusU]

@SimenB ah right, no it won't. Expo currently doesn't have a sync random api, opening upstream issue now

@grit96 is that on Expo?

[geraintwhite]

@LinusU nope, RN 0.59. I just realised that react-native-get-random-values probably needs linking because I'm using RN < 0.60.

[LinusU]

probably needs linking because I'm using RN < 0.60

Yeah, that's probably it.

Sorry, I removed the linking step from the readme since I thought that most people would be on >= 0.60 by now ☺️

[LinusU]

Issue filed with Expo here: expo/expo#7209