feat(armstrong_number): new optional exercise for rust piscine

subjects/armstrong_number/README.md

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+## armstrong_number
+
+### Instructions
+
+Create a function which checks if the number is a valid Armstrong number. The function will return the number if it is a valid one and `None` otherwise.
+
+An Armstrong number is a number where the sum of its own digits each raised to the power of the number of digits is equal to itself.
+
+As an example 153 is an Armstrong number because:
+`153 = 1^3 + 5^3 + 3^3`
+
+### Expected Function
+
+```rust
+pub fn is_armstrong_number(nb: u32) -> Option<u32> {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function,
+
+```rust
+fn main() {
+    println!("{:?}", is_armstrong_number(0));
+    println!("{:?}", is_armstrong_number(1));
+    println!("{:?}", is_armstrong_number(153));
+    println!("{:?}", is_armstrong_number(370));
+    println!("{:?}", is_armstrong_number(371));
+    println!("{:?}", is_armstrong_number(407));
+    println!("{:?}", is_armstrong_number(400));
+    println!("{:?}", is_armstrong_number(198));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+Some(0)
+Some(1)
+Some(153)
+Some(370)
+Some(371)
+Some(407)
+None
+None
+$
+```