Exporting to Android: explain limits/scope of debug keystore, how to make release one

See https://www.facebook.com/groups/godotengine/permalink/1366053016864622/

In http://docs.godotengine.org/en/latest/getting_started/workflow/export/exporting_for_android.html we give instructions to generate a debug keystore, which is something that is good enough to test your game on your phone, but not meant for use in production. The limits of this debug keystore should be better explained (invalid certificate - see company name used -, enormous validity span, etc.).

The page should also document how to create a *valid* release keystore (and maybe also a valid debug keystore if relevant) for shipping games in production. It can likely refer to https://developer.android.com/studio/publish/app-signing#generate-key