Add +nightly to command for running cargo build.

Author: Darren Tsung

README.md

@@ -47,7 +47,7 @@ And, in your `main.rs` or `lib.rs`, add these lines:
 #![cfg_attr(feature="clippy", plugin(clippy))]
 ```
 
-Then build by enabling the feature: `cargo build --features "clippy"`
+Then build by enabling the feature: `cargo +nightly build --features "clippy"`.
 
 Instead of adding the `cfg_attr` attributes you can also run clippy on demand:
 `cargo rustc --features clippy -- -Z no-trans -Z extra-plugins=clippy`