React-native integration doc could point to simple start functions as well

[aureosouza]

We noticed that the documentation for Android and iOS presents the possibilty of starting the services calling start functions that accept the appSecret as an argument.

But this is not particularly true for the React Native doc, which expects the AppCenter-Config.plist and appcenter-config.json files to be added for each build flavor. This is an extra complexity when we have multiple build flavors configured in AppCenter. Or when we'd like to integrate with known solutions such as react-native-config which uses a single source of truth file for all build flavor specific configs. And in fact there's a few issues regarding app center opened there (here, here, and here) and the workaround so far was to use pre-build to generate these files on the fly, which doesn't seem like an optimal solution.

The point is in fact we can call the native start functions with the appSecret in React Native integration as well. So if we have already a solution in place for build flavors configs, we could pass the appSecrets as argument using React Native in the same way as the regular Android and iOS documentation points out. Writed a small post on this here.

So a possible suggestion could be to present this option in the React Native doc as well, in which it's not required necessarly to have multiple AppCenter-Config.plist and params in appcenter-config.json, but we could call the Android start on MainApplication and the iOS start or configure in AppDelegate when integrating react native projects.