fix(intro1): Add compiler error explanation.

exercises/intro/intro1.rs

@@ -16,4 +16,8 @@ fn main() {
     println!(r#" | |  | |_| \__ \ |_| | | | | | (_| \__ \ "#);
     println!(r#" |_|   \__,_|___/\__|_|_|_| |_|\__, |___/ "#);
     println!(r#"                               |___/      "#);
+    println!();
+    println!("This exercise compiles successfully. The remaining exercises contain a compiler");
+    println!("or logic error. The central concept behind Rustlings is to fix these errors and");
+    println!("solve the exercises. Good luck!");
 }