Communicate better the debug purpose of Reqres

[olejnjak]

As we internally reconsidered Reqres to be debug tool that should not be used in production, we never made this reconsideration public (because of #33).

To make this clear we should:

• definitely add this statement to README
• we can add a notice to console when Reqres is configured
• probably we could be able to check if we are running in app downloaded from App Store and disable Reqres ourselves