add chapter [println and format]

solutions/formatted-output/println.md

@@ -0,0 +1,18 @@
+1.
+```rust
+fn main() {
+    let s1 = "hello";
+    /* Fill in the blank */
+    let s = format!("{}, world!", s1);
+    assert_eq!(s, "hello, world!");
+}
+```
+
+2.
+```rust
+fn main() {
+    print!("hello world, ");
+    println!("I am");
+    println!("Sunface!");
+}
+```