add solutions for variables

src/ownership/borrowing.md

@@ -163,4 +163,6 @@ fn main() {
     // add one line below to make a compiler error: cannot borrow `s` as mutable more than once at a time
     // you can't use r1 and r2 at the same time
 }
-```
+```
+
+> 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