udpate dataset download instructions

data/get_coco2017.sh

 #!/bin/bash
-# Zip coco folder
-# zip -r coco.zip coco
-# tar -czvf coco.tar.gz coco
+# COCO 2017 dataset http://cocodataset.org
+# Download command: bash yolov5/data/get_coco2017.sh
+# Train command: python train.py --data ./data/coco.yaml
+# Dataset should be placed next to yolov5 folder:
+#   /parent_folder
+#     /coco
+#     /yolov5
 
 # Download labels from Google Drive, accepting presented query
 filename="coco2017labels.zip"

data/get_voc.sh

+# PASCAL VOC dataset http://host.robots.ox.ac.uk/pascal/VOC/
+# Download command: bash yolov5/data/get_voc.sh
+# Train command: python train.py --data voc.yaml
+# Dataset should be placed next to yolov5 folder:
+#   /parent_folder
+#     /VOC
+#     /yolov5
+
 start=`date +%s`
 
 # handle optional download dir