Improve README of integration of CodePush

[max-mironov]

Intent: As of now the latest versions of React Native have the following logic to export the class by default:

export default class App extends Component<{}> {}

This will not work in some cases for CodePush as we need to export the class after we've codepushified the app (#1147)
So added explicit export examples for docs to make the intentions clear.

@Zakeelm probably we also need to update docs here: https://docs.microsoft.com/en-us/appcenter/distribution/codepush/react-native

[sergey-akhalkov]

LGTM