Traits - Simple Tree - Add migration (#230)

Co-authored-by: Marc Jauvin <[email protected]>

Author: WebVPF

database/traits.md

@@ -207,7 +207,15 @@ $user->setSortableOrder([1, 2, 3], [3, 2, 1]);
 
 ## Simple Tree
 
-A simple tree model will use the `parent_id` column maintain a parent and child relationship between models. To use the simple tree, apply the `Winter\Storm\Database\Traits\SimpleTree` trait.
+A simple tree model will use the `parent_id` column maintain a parent and child relationship between models. To add the `parent_id` column to your table, you may use the `unsignedInteger` method inside a migration.
+
+```php
+Schema::table('categories', function ($table) {
+    $table->unsignedInteger('parent_id')->nullable();
+});
+```
+
+To use the simple tree, apply the `Winter\Storm\Database\Traits\SimpleTree` trait.
 
 ```php
 class Category extends Model