Implement add function in Subscriber repository.

src/repository/notification.rs

@@ -9,4 +9,9 @@ use lazy_static::lazy_static;
  
  pub struct NotificationRepository;
  
- impl NotificationRepository {}
+ impl NotificationRepository {
\ No newline at end of file
+    pub fn add(notification: Notification) -> Notification {
+        NOTIFICATIONS.write().unwrap().push(notification.clone());
+        return notification;
+    }
+ }
\ No newline at end of file