Add a document on how to implement check-in system

BACKLOG.md

@@ -3,10 +3,6 @@
 ## 📋 Todo
 
 ### High Priority
-- [ ] feat: Payment gateway integration (Stripe, PayPal)
-- [ ] feat: Digital tickets with QR codes
-- [ ] feat: Ticket inventory management and capacity limits
-- [ ] feat: Event discovery with search and filtering
 - [ ] feat: Check-in system with QR code scanning
 
 ### Medium Priority
@@ -35,7 +31,6 @@
 
 ## 🚧 Doing
 
-- [ ] feat: Implement user dashboard to display past events, tickets and orders
 
 ## ✅ Done
 
@@ -43,4 +38,8 @@
 - [x] Create authentication system
 - [x] Implement user registration
 - [x] Add login functionality
-- [x] refactor: Moving checkout to OrdersController
+- [x] refactor: Moving checkout to OrdersController
+- [x] feat: Payment gateway integration (Stripe) - PayPal not implemented
+- [x] feat: Digital tickets with QR codes
+- [x] feat: Ticket inventory management and capacity limits
+- [x] feat: Event discovery with search and filtering