docs: update readme

Author: sinedied

README.md

@@ -184,7 +184,7 @@ The Burger MCP server provides these tools for AI agents:
 | `get_topping_categories` | Get a list of all topping categories                                                         |
 | `get_orders`             | Get a list of all orders in the system                                                       |
 | `get_order_by_id`        | Get a specific order by its ID                                                               |
-| `place_order`            | Place a new order with burgers (requires `userId`)                                           |
+| `place_order`            | Place a new order with burgers (requires `userId`, optional `nickname`)                                           |
 | `delete_order_by_id`     | Cancel an order if it has not yet been started (status must be `pending`, requires `userId`) |
 
 ### Testing the MCP Server