Added setRowId description

Added the setRowId documentation mentioned in yajra/laravel-datatables#2661 (comment)

Author: schonhoff

index-column.md

@@ -16,4 +16,20 @@ Route::get('user-data', function() {
 ```
 
 Using `addIndexColumn` will add another column on your response with a column name that is set on [`index_column`](/docs/{{package}}/{{version}}/general-settings#index-column) configuration.
-The default index column name is `DT_RowIndex`
+The default index column name is `DT_RowIndex`.
+
+If you want to customize the index used by the `DT_RowIndex`, you can use `setRowId('COLUMN')` to change the index number.
+
+```php
+use DataTables;
+
+Route::get('user-data', function() {
+	$model = App\User::query();
+
+	return DataTables::eloquent($model)
+				->setRowId('id')
+				->toJson();
+});
+```
+
+Be careful with this option, an index should be unique and an integer to be useful for DataTables.