feat(scytale_decoder): add new exercise for rust exams

subjects/scytale_decoder/README.md

@@ -0,0 +1,64 @@
+## scytale_cipher
+
+### Instructions
+
+Create a **function** which decode a scytale cipher (also known as spartan cipher).
+
+In practice, it is represented by a strip wrapped around a cylinder. The message is written across the loops of the strip (not along the strip). The message becomes *coded* if the radius of the cylinder changes, or the strip is removed from the cylinder.
+
+Your function will receive a `String` representing the ciphered message, and a `u32` representing the number of letters by turn of the strip around the cylinder.
+
+> If the ciphered message is empty or the letters per turn are 0 the function will return `None`.
+
+### Example
+
+**letters_per_turn 2:** `"scytale Code"` -> `"sec yCtoadle"`
+
+```console
+--------------------------------
+  |s|  |c|  |y|  |t|  |a|  |l|
+  |e|  | |  |C|  |o|  |d|  |e|
+--------------------------------
+```
+
+**letters_per_turn 4:** `"scytale Code"` -> `"steoca dylCe"`
+
+
+```console
+------------------------------------------
+  |s|  |c|  |y|
+  |t|  |a|  |l|
+  |e|  | |  |C|
+  |o|  |d|  |e|
+------------------------------------------
+```
+
+### Expected Functions
+
+```rust
+pub fn scytale_decoder(s: String, letters_per_turn: u32) -> Option<String> {
+}
+```
+
+### Usage
+
+Here is a program to test your function
+
+```rust
+fn main() {
+    println!("\"sec yCtoadle\" size=2 -> {:?}",
+        scytale_decoder("sec yCtoadle".to_string(), 2));
+
+    println!("\"steoca dylCe\" size=4 -> {:?}",
+        scytale_decoder("steoca dylCe".to_string(), 4));
+}
+```
+
+And its output
+
+```console
+$ cargo run
+"sec yCtoadle" size=2 -> Some("scytale Code")
+"steoca dylCe" size=4 -> Some("scytale Code")
+$
+```