[Basic #2. Hashes] Mention how to deserialize before exercise

[matthabermehl]

Up to this point, the retrieval of an element, and getting an entry hasn't been discussed, so the line in the solution

let book_option: Option<Book> = element.entry().to_app_option()?;

was unexpected. Is it possible, before the exercise, to include a section on how we get an element, gets its entry, and deserialize?