Update README with due date functionality documentation

Added due date field documentation to Todo model and included usage examples
for creating todos with due dates and timezone handling.

README.md

@@ -108,9 +108,16 @@ Each todo has the following fields:
 - `completed`: Completion status (boolean, default: false)
 - `priority`: Priority level (low, medium, high, default: medium)
 - `category_id`: Reference to category (optional)
+- `project_id`: Reference to project (optional)
+- `due_date`: Due date and time with timezone (optional)
+- `parent_id`: Reference to parent todo for subtasks (optional)
 - `created_at`: Creation timestamp
 - `updated_at`: Last update timestamp
 
+#### Due Date Features
+- `is_overdue`: Computed property indicating if todo is past due date
+- `days_until_due`: Computed property showing days until due (negative if overdue)
+
 ### Category Model
 Each category has the following fields:
 - `id`: Unique identifier (auto-generated)
@@ -136,6 +143,13 @@ curl -X POST "http://localhost:8000/api/v1/todos" \
      -d '{"title": "Review presentation", "description": "Review the quarterly presentation", "category_id": 1, "priority": "high"}'
 ```
 
+### Creating a Todo with Due Date
+```bash
+curl -X POST "http://localhost:8000/api/v1/todos" \
+     -H "Content-Type: application/json" \
+     -d '{"title": "Submit report", "description": "Submit quarterly report", "due_date": "2024-12-31T23:59:59Z", "priority": "high"}'
+```
+
 ### Filtering Todos by Category
 ```bash
 curl "http://localhost:8000/api/v1/todos?category_id=1"