add README (disable viewer, document structure)

README.md

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+---
+viewer: false
+tags:
+  - adversarial-robustness
+  - image-classification
+  - robustness-benchmark
+---
+
+# RobustGenBench-sample
+
+This is a **stratified sample** of the full [RobustGenBench](https://huggingface.co/datasets/legolasflagstaff/RobustGenBench) dataset (22 GB), created for NeurIPS dataset submission review.
+
+## Structure
+
+Each subfolder contains **2 images per class** extracted from the corresponding archive in the full dataset, plus `test/labels.csv` and `metadata.json`.
+
+```
+clean/
+  caltech101/          ← 2 imgs/class × 101 classes = 202 images
+  fgvc-aircraft-2013b/
+  flowers-102/
+  oxford-iiit-pet/
+  stanford_cars/
+  uc-merced-land-use-dataset/
+
+adversarial/
+  common/common_severity3/<dataset>/
+  random/linf_eps30_random_uniform/<dataset>/
+  zeroshot_clip_vitb16_laion2b/<threat_model>/<dataset>/
+  zeroshot_clip_vith14_laion2b/<threat_model>/<dataset>/
+  zeroshot_metaclip_vith14_fullcc2_5b/<threat_model>/<dataset>/
+  zeroshot_siglip2_base_patch16_224/<threat_model>/<dataset>/
+  zeroshot_siglip2_so400m_patch14_384/<threat_model>/<dataset>/
+  zeroshot_siglip2_so400m_patch16_naflex/<threat_model>/<dataset>/
+  zeroshot_siglip2_so400m_patch16_naflex_patchify/<threat_model>/<dataset>/
+```
+
+Each leaf folder has the same internal layout:
+- `test/labels.csv` — `filename,label` mapping (integer class indices)
+- `test/NNNNN.png` — flat-numbered PNG images (same filenames across clean & adversarial)
+- `metadata.json` — split counts (clean archives only)
+
+Class name mappings are in `class_names/<dataset>.json`.
+
+See **[SAMPLE.md](SAMPLE.md)** for full details on sampling methodology.
+
+## Full dataset
+
+👉 https://huggingface.co/datasets/legolasflagstaff/RobustGenBench