missing import `#[cfg(test)] use mockers_derive::mocked;` in guide.md example

[mrcosta]

It took some time to open https://github.com/kriomant/mockers/blob/master/examples/air_proc_macro and see what is missing in my implementation. Without this import I get the following error:

let mut cond = scenario.create_mock_for::<AirConditioner>();
   |                                 ^^^^^^^^^^^^^^^
   |
   = note: the method `create_mock_for` exists but the following trait bounds were not satisfied:
           `&'static _ : mockers::Mocked`
   = help: did you mean `create_mock`?

[mrcosta]

I added #41
Sorry but I forgot to mention the issue number in the commit.