add documentation

src/main.rs

@@ -59,6 +59,7 @@ fn create_hash_map(pool: &Vec<robot::Robot>, hash: &mut HashMap<robot::Position,
     }
 }
 
+/// Generate random instructions.
 fn gen_random_instructions() -> String {
     let mut rng = rand::thread_rng();
     let n = rng.gen_range(5, 10);