edit README.md: add folder for download_script

README.md

@@ -14,6 +14,8 @@ organized by competition. Contains high quality data, prioritizing "official" so
 ├── raw/               # Raw problem/solution statements (.pdf)
 │   ├── file1.pdf
 │   ├── file2.pdf
+├── download_script/   # the scripts used to download raw data
+│   ├── download.py    
 ├── md/                # .md files generated from raw/ files
 │   ├── file1.md
 │   ├── file2.md