Mention selective importing in 'rocket_codegen' docs.

core/codegen/src/lib.rs

@@ -26,7 +26,8 @@
 //! rocket = "0.4.0-dev"
 //! ```
 //!
-//! And to import macros via `#[macro_use]` in the crate root:
+//! And to import all macros, attributes, and derives via `#[macro_use]` in the
+//! crate root:
 //!
 //! ```rust
 //! #![feature(proc_macro_hygiene, decl_macro)]
@@ -36,6 +37,17 @@
 //! # fn main() { rocket::ignite().mount("/", routes![hello]); }
 //! ```
 //!
+//! Or, alternatively, selectively import from the top-level scope:
+//!
+//! ```rust
+//! #![feature(proc_macro_hygiene, decl_macro)]
+//! # extern crate rocket;
+//!
+//! use rocket::{get, routes};
+//! # #[get("/")] fn hello() { }
+//! # fn main() { rocket::ignite().mount("/", routes![hello]); }
+//! ```
+//!
 //! # Debugging Codegen
 //!
 //! When the `ROCKET_CODEGEN_DEBUG` environment variable is set, this crate