chore: added helpful instruction comment on lable_image sample (#52)

Fixes b/172749007 🦕

Author: munkhuushmgl

datalabeling/snippets/label_image.py

@@ -45,6 +45,9 @@ def label_image(
 
     feature = datalabeling.LabelImageRequest.Feature.CLASSIFICATION
 
+    # annotation_spec_set_resource_name needs to be created beforehand.
+    # See the examples in the following:
+    # https://cloud.google.com/ai-platform/data-labeling/docs/label-sets
     config = datalabeling.ImageClassificationConfig(
         annotation_spec_set=annotation_spec_set_resource_name,
         allow_multi_label=False,