Create README.md

README.md

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+These are the idx files used by the `MMapIndexedDataset` used to load the memory-mapped Pile dataset. 
+To load the dataset, you can follow the instructions in the [README.md](https://github.com/EleutherAI/pythia/tree/main?tab=readme-ov-file#reproducing-training)
+file in the Pythia GitHub repo. Then, for each seed, you can simply pass the paths to the idx files. The `dataset.py` file provides an example of how to do that.