Add `use test_case::test_case;` to CONTRIBUTING.md

I previously tried adding the #[test_case()] attribute macro and got
confused because the Rust compilation suddenly failed with:

    error[E0658]: use of unstable library feature 'custom_test_frameworks': custom test frameworks are an unstable feature

which is a quite confusing error message.  The solution is to just add
`use test_case::test_case;`, so this commit adds that line to the
example in CONTRIBUTING.md to spare others this source of confusion.

CONTRIBUTING.md

@@ -71,6 +71,8 @@ output, codify the behavior as a snapshot test by adding a new `testcase` macro
 section of `src/linter.rs`, like so:
 
 ```rust
+use test_case::test_case;
+
 #[test_case(CheckCode::A001, Path::new("A001.py"); "A001")]
 ...
 ```