Add hints to the variable bindings exercises

variables/variables2.rs

@@ -1,4 +1,4 @@
-// Make me compile!
+// Make me compile! Scroll down for hints :)
 
 fn main() {
     let x;
@@ -8,3 +8,39 @@ fn main() {
         println!("Not ten!");
     }
 }
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+// The compiler message is saying that Rust cannot infer the type that the
+// variable binding `x` has with what is given here.
+// What happens if you annotate line 4 with a type annotation?
+// What if you give x a value?
+// What if you do both?
+// What type should x be, anyway?
+// What if x is the same type as 10? What if it's a different type?