Add hints to the variable bindings exercises

variables/variables3.rs

@@ -1,4 +1,4 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x = 3;
@@ -6,3 +6,37 @@ fn main() {
     x = 5;
     println!("Number {}", x);
 }
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+// In Rust, variable bindings are immutable by default. But here we're trying
+// to reassign a different value to x! There's a keyword we can use to make
+// a variable binding mutable instead.