Add guide for Testflight and Android internal, alpha, and beta testing

[coreyshuman]

Getting an app setup in Android or Apple for internal and external testing takes Herculean effort. We should document this process on each platform. Screenshots would definitely be a plus, given how difficult both developer consoles are to navigate. Finally, adding a place for gotchas would be great (such as subscriptions not being available in iOS for testing if the client hasn't signed their tax agreement yet).