Update readme with category ID details

test_data/README.md

@@ -28,8 +28,8 @@ A small size bounding boxes dataset in VIA, COCO and VIA-CSV formats.
 
 | File name | Type | Format | N annotations | N images | Comments |
 | --- | --- | --- | --- | --- | --- |
-| `small_bboxes_VIA.json` | bboxes | VIA | 3 | 3 | One bounding box per frame. |
-| `small_bboxes_COCO.json` | bboxes | COCO | 3 | 3 | One bounding box per frame. |
+| `small_bboxes_VIA.json` | bboxes | VIA | 3 | 3 | One bounding box per frame. The category ID is "1" (string) for all annotations. |
+| `small_bboxes_COCO.json` | bboxes | COCO | 3 | 3 | One bounding box per frame. The category ID is 1 (integer) for all annotations. |
 
 #### `small_bboxes_dataset_duplicates`