Add warning about usecase of labelme_json_to_dataset

scripts/labelme_json_to_dataset

@@ -13,6 +13,11 @@ import yaml
 from labelme import utils
 
 
+warnings.warn("This script is aimed to demonstrate how to convert the\n"
+              "JSON file to a single image dataset, and not to handle\n"
+              "multiple JSON files to generate a real-use dataset.")
+
+
 def main():
     parser = argparse.ArgumentParser()
     parser.add_argument('json_file')