luhn-from: Added template to the stub file (#613)

Contributes to #551

Author: ZapAnton

exercises/luhn-from/.meta/ALLOWED_TO_NOT_COMPILE

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

exercises/luhn-from/src/lib.rs

@@ -1 +1,18 @@
+pub struct Luhn;
 
+impl Luhn {
+    pub fn is_valid(&self) -> bool {
+        unimplemented!("Determine if the current Luhn struct contains a valid credit card number.");
+    }
+}
+
+/// Here is the example of how the From trait could be implemented
+/// 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> From<&'a str> for Luhn {
+    fn from(input: &'a str) -> Self {
+        unimplemented!("From the given input '{}' create a new Luhn struct.", input);
+    }
+}