Add example for shared_state

[cuzfrog]

Reading this simple and comprehensive book, while finding the example for shared_shate is not complete. Hope this added simple example could serve.

[mgeisler]

Hi @cuzfrog,

You have actually solved the exercise which I want to solve with the students in class 😄

In other words, the example doesn't compile because I want the students to "help" me fix it during class. That's obviously not very clear right now — I'm working on adding support for speaker notes #53 and then I'll describe the solution in the notes. That should make the course much better for self-study.

I hope that makes sense!