add simple io.

src/main.rs

@@ -1,3 +1,12 @@
+use std::io;
+
 fn main() {
-    println!("Hello, world!");
+    println!("Guessing a number:");
+
+    let mut guess_number = String::new();
+
+    io::stdin().read_line(&mut guess_number)
+        .expect("Failed to read line.");
+
+    println!("You guess : {}", guess_number)
 }