Add docs on downloading LAION-COCO

dataset_examples/laion-coco.md

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+## LAION-COCO
+
+LAION-COCO is a 600M subset of LAION2B-EN, captioned with an ensemble of BLIP L/14 and 2 CLIP versions (L/14 and RN50x64).
+It is available at https://huggingface.co/datasets/laion/laion-coco
+
+### Download the metadata
+
+Download from https://huggingface.co/datasets/laion/laion-coco
+
+```bash
+mkdir -p laion-coco && cd laion-coco/
+
+for i in {0..127}; do 
+    wget "https://huggingface.co/datasets/laion/laion-coco/resolve/main/part-$(printf "%05d" $i)-2256f782-126f-4dc6-b9c6-e6757637749d-c000.snappy.parquet"
+done
+
+cd ..
+```
+
+### Download the images with img2dataset
+
+```bash
+img2dataset --url_list laion-coco --input_format "parquet"\
+         --url_col "URL" --caption_col "TEXT" --output_format webdataset\
+           --output_folder laion-coco-output --processes_count 16 --thread_count 64 --image_size 256\
+            --resize_only_if_bigger=True --resize_mode="keep_ratio" --skip_reencode=True \
+             --save_additional_columns '["similarity","hash","punsafe","pwatermark","top_caption","all_captions","all_similarities"]' --enable_wandb True
+```