feat: Add move_semantics6.rs exercise (#908)

exercises/move_semantics/move_semantics6.rs

@@ -0,0 +1,25 @@
+// move_semantics6.rs
+// Make me compile! `rustlings hint move_semantics6` for hints
+// You can't change anything except adding or removing references
+
+// I AM NOT DONE
+
+fn main() {
+    let data = "Rust is great!".to_string();
+
+    get_char(data);
+
+    string_uppercase(&data);
+}
+
+// Should not take ownership
+fn get_char(data: String) -> char {
+    data.chars().last().unwrap()
+}
+
+// Should take ownership
+fn string_uppercase(mut data: &String) {
+    data = &data.to_uppercase();
+
+    println!("{}", data);
+}