[book] When adding files to XCode make sure to tell newbies to "reference files" and *not* copy them

[kshehata]

This tripped me up following the Xcode + Cargo instructions, I just accepted the defaults, as below.

This made a copy of the files in the project, disconnected from the generated files. Instead, you need to choose "reference files in place":

This would also be a lot easier if the first half of the tutorial used the Rust code to do something, so that if there's a mistake made the rust import is empty it doesn't silently accept it.