Add a story to the luhn exercise (#2519)

* add a story

* update instructions

Author: tasxatzial

exercises/luhn/description.md renamed to exercises/luhn/instructions.md

@@ -1,12 +1,10 @@
-# Description
+# Instructions
 
-Given a number determine whether or not it is valid per the Luhn formula.
+Determine whether a credit card number is valid according to the [Luhn formula][luhn].
 
-The [Luhn algorithm][luhn] is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers and Canadian Social Insurance Numbers.
+The number will be provided as a string.
 
-The task is to check if a given string is valid.
-
-## Validating a Number
+## Validating a number
 
 Strings of length 1 or less are not valid.
 Spaces are allowed in the input, but they should be stripped before checking.

exercises/luhn/introduction.md

@@ -0,0 +1,11 @@
+# Introduction
+
+At the Global Verification Authority, you've just been entrusted with a critical assignment.
+Across the city, from online purchases to secure logins, countless operations rely on the accuracy of numerical identifiers like credit card numbers, bank account numbers, transaction codes, and tracking IDs.
+The Luhn algorithm is a simple checksum formula used to ensure these numbers are valid and error-free.
+
+A batch of identifiers has just arrived on your desk.
+All of them must pass the Luhn test to ensure they're legitimate.
+If any fail, they'll be flagged as invalid, preventing errors or fraud, such as incorrect transactions or unauthorized access.
+
+Can you ensure this is done right? The integrity of many services depends on you.