Add dataset sizes (zipped) (#7293)

data/VisDrone.yaml

@@ -4,7 +4,7 @@
 # parent
 # ├── yolov5
 # └── datasets
-#     └── VisDrone  ← downloads here
+#     └── VisDrone  ← downloads here (2.3 GB)
 
 
 # Train/val/test sets as 1) dir: path/to/imgs, 2) file: path/to/imgs.txt, or 3) list: [path/to/imgs1, path/to/imgs2, ..]