Update download_weights.sh with usage example (#1613)

weights/download_weights.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 # Download latest models from https://github.com/ultralytics/yolov5/releases
+# Usage:
+#    $ bash weights/download_weights.sh
 
 python - <<EOF
 from utils.google_utils import attempt_download