Add HRRR idx-only smoke-test download option

data_downloader/README.md

@@ -88,6 +88,19 @@ The HRRR downloader retrieves native HRRR files; county, sub-county, or any
 other custom spatial granularity should be produced later by the serving or
 preprocessing layer using user-provided boundaries.
 
+For a small real-download smoke test, download only the HRRR index sidecar:
+
+```bash
+python3 data_downloader/hrrr_downloader.py \
+  --start-date 2024-06-01 \
+  --end-date 2024-06-01 \
+  --hours 00 \
+  --forecast-hours 00 \
+  --idx-only \
+  --max-files 1 \
+  --output-root ./downloads_hrrr_idx_test
+```
+
 ## Implemented Direct Downloads
 
 These sources are implemented as direct public downloads: