Update README.md

README.md

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 English
 
+## Data Fields
+
+There are 4 fields: `images`, `texts`, `metadata` and `general_metadata`.
+
+For each example, the data in the columns `images` and `texts` are two lists of the same size, where for each index, one element and only one is not `None`.
+
+For example, for the web document `<image_1>text<image_2>`, in `images`, we have `[image_1,None,image_2]` and in `texts` we have `[None,text,None]`.
+
+The images are replaced by their URLs, and the users have to download them themselves, for example with the library `img2dataset`.
+
+In `metadata`, there is a string that can be transformed into a list with `json.loads(example["metadata"])`. This list will have the same size as the lists of images and texts, and will have a dictionary for each index where there is an image, and a `None` value when there is a text. This dictionary will contain the metadata of the images (original source document, unformatted source, alt-text if present, ...).
+
+Finally, in `general_metadata`, there is a string that can be transformed into a dictionary, containing the URL of the document, and information about its location in the Common Crawl data.
+
 ### Visualization of OBELISC documents
 
 https://huggingface.co/spaces/HuggingFaceM4/obelisc_visualization