Add an introductory tutorial for ScopedServices and state sharing between screens

[Zhuinden]

As described by author of https://github.com/isaac-udy/Enro/.

Technically I have one in https://github.com/Zhuinden/simple-stack/tree/master/simple-stack-example-scoping that is 2 years old and I should probably still create one here too, probably one that's slightly nicer in its structure.