[book] Add note to use ffi

[jreidinger]

Hi,
at first let me thanks you for this nice project that is quite useful. I use it to setup some first draft for using gobject API from rust for my project and when I follow instructions in book and failed to compile it. After debugging I found that use ffi; in lib.rs solve my issue with generated code and I think it should be part of book so others do not need to face it.
Thanks

[bilelmoussaoui]

there are probably other things that should be added there but this seems important enough to merge it for now
thanks!

[jreidinger]

Thanks for merging it. I also consider if I should add pub use auto::traits::*; which I also needs for interfaces, but it is partly covered in handling errors and there is suggested solution to use prelude, so I go with minimal patch which is critical to make basically any generated code works.