Document need for #[macro_use] for traits in modules

[hobofan]

Follow up to #18 (comment).

There should be a part in the README that specifies the need for #[macro_use if you want to use traits across modules.

Would also be interesting to know if we can add a compile-time check if the macro is available (probably not, as it's really early in the compilation), and emit a error message with the most common pitfalls.