add solutions for variables

src/variables.md

@@ -143,4 +143,7 @@ fn main() {
     // fill the blank to make the code work
     assert_eq!([x,y], __);
 } 
-```
+```
+
+
+> You can find the solutions [here](https://github.com/sunface/rust-by-practice)(under the solutions path), but only use it when you need it