feat(count_factorial_steps): add new rust exercise

subjects/count_factorial_steps/README.md

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+## count_factorial_steps
+
+### Instructions
+
+Create a **function** named `count_factorial_steps` that receives a factorial number and counts how many multiplications are necessary to have this number.
+
+If the argument is not a factorial or == 0 || 1 then the function should return 0.
+
+```rust
+fn count_factorial_steps(factorial: u64) -> u64 {
+}
+```
+
+As a reminder, the factorial of a number is the product of all the integers from 1 to that number.
+
+Example: the factorial of 6 (written 6!) is 1 \* 2 \* 3 \* 4 \* 5 \* 6 = 720.
+
+### Usage
+
+Here is a possible program to test your function:
+
+```rust
+use count_factorial_steps::count_factorial_steps;
+
+fn main() {
+    println!("The factorial steps of 720 = {}", count_factorial_steps(720));
+    println!("The factorial steps of 13 = {}", count_factorial_steps(13));
+    println!("The factorial steps of 6 = {}", count_factorial_steps(6));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+The factorial steps of 720 = 6
+The factorial steps of 13 = 0
+The factorial steps of 6 = 3
+```