add YFCC100M subset information

(openai#50)

data/yfcc100m.md

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+# The YFCC100M Subset
+
+In the paper, we performed a dataset ablation using a subset of the YFCC100M dataset and showed that the performance remained largely similar. 
+
+The subset contains 14,829,396 images, about 15% of the full dataset, which have been filtered to only keep those with natural languag titles and/or descriptions in English.
+
+We provide the list of (line number, photo identifier, photo hash) of each image contained in this subset. These correspond to the first three columns in the dataset's metadata TSV file.
+
+```
+wget https://openaipublic.azureedge.net/clip/data/yfcc100m_subset_data.tsv.bz2
+bunzip2 yfcc100m_subset_data.tsv.bz2
+```
+
+Use of the underlying media files is subject to the Creative Commons licenses chosen by their creators/uploaders. For more information about the YFCC100M dataset, visit [the official website](https://multimediacommons.wordpress.com/yfcc100m-core-dataset/).