Add solutions to intro and variables

solutions/01_variables/variables5.rs

@@ -1 +1,9 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+fn main() {
+    let number = "T-H-R-E-E"; // Don't change this line
+    println!("Spell a number: {}", number);
+
+    // Using variable shadowing
+    // https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#shadowing
+    let number = 3;
+    println!("Number plus two is: {}", number + 2);
+}