luhn-trait: Add template to the stub file (#614)

Contributes to #551

Author: ZapAnton

exercises/luhn-trait/.meta/ALLOWED_TO_NOT_COMPILE

@@ -0,0 +1,2 @@
+Stub doesn't compile because the custom trait is not implemented for the every type in the test suite.
+This exercise is an introduction to implementing custom traits for the primitives and fully implementing the trait for the test suite reduces learning.

exercises/luhn-trait/src/lib.rs

@@ -1 +1,14 @@
+pub trait Luhn {
+    fn valid_luhn(&self) -> bool;
+}
 
+/// Here is the example of how to implement custom Luhn trait
+/// for the &str type. Naturally, you can implement this trait
+/// by hand for the every other type presented in the test suite,
+/// but your solution will fail if a new type is presented.
+/// Perhaps there exists a better solution for this problem?
+impl<'a> Luhn for &'a str {
+    fn valid_luhn(&self) -> bool {
+        unimplemented!("Determine if '{}' is a valid credit card number.", self);
+    }
+}