udpate dataset download instructions

data/get_voc.sh

@@ -1,3 +1,11 @@
+# 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