feat(luhn_algorithm): new optional exercise for rust piscine

subjects/luhn_algorithm/README.md

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+## luhn_algorithm
+
+### Instructions
+
+Create a function which checks if a number is valid per the Luhn formula.
+
+The function will receive a string and return a boolean.
+An empty string or a number with only one digit will be considered an invalid number.
+Spaces are accepted in the script but have to be stripped during the calculation (in other words they won't affect the result).
+
+The Luhn formula is used to check if a number is a valid credit card number and in some other scenarios where you need to check a number fast and without accessing a database.
+
+We can summarize the formula as follow:
+
+- We want to check the number `4539 3195 0343 6467`
+- We take every second digit starting by the right
+- We multiply those digits by 2
+- If the result is more than 9 we subtract 9 from it
+- We sum all the digits
+- If sum is evenly divisible by 10 then this number is valid
+
+So we will get:
+
+- `4539 3195 0343 6467`
+- `4_3_ 3_9_ 0_4_ 6_6_`: numbers to modify
+- `8_6_ 6_9_ 0_8_ 3_3_`: modified numbers
+- `8569 6195 0383 3437`: the new sequence of digits
+- `80`: the sum of all digits
+- `80` is evenly divisible by 10 so the result is `true`
+
+### Expected Function
+
+```rust
+pub fn is_luhn_formula(code: &str) -> bool {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function,
+
+```rust
+fn main() {
+    println!("{}", is_luhn_formula(""));
+    println!("{}", is_luhn_formula("1"));
+    println!("{}", is_luhn_formula("79927398713"));
+    println!("{}", is_luhn_formula("7992 7398 713"));
+    println!("{}", is_luhn_formula("1234567890123456"));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+false
+false
+true
+true
+false
+$
+```