Add an example of the toplevel_ref_arg lint.

examples/toplevel_ref_arg.rs

@@ -0,0 +1,14 @@
+#![feature(phase)]
+
+#[phase(plugin)]
+extern crate rust_clippy;
+
+fn the_answer(ref mut x: u8) {
+  *x = 42;
+}
+
+fn main() {
+  let mut x = 0;
+  the_answer(x);
+  println!("The answer is {}.", x);
+}