Actually adds a README

README.md

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+This is the processed version of [Google's C4 dataset](https://www.tensorflow.org/datasets/catalog/c4).
+
+We prepared three variants of the data: `en`, `en.noclean`, and `realnewslike`. There is a fourth one, `webtextlike`, which was not ready as I am writing this, but we are working on it. If you are interested in the `multilingual` version, please get in touch.
+
+For reference, these are the sizes of the variants:
+
+- `en`: 300GB
+- `en.noclean`: 2.3TB in JSON format
+- `realnewslike`: 15GB in JSON format
+
+# How do I download this?
+
+Unfortunately we ran out of time making this into a proper Huggingface dataset, accessible through the `datasets` Python package. Until we get that ready, please use git to do the download. First, make sure you have [Git Large File Storage](https://git-lfs.github.com) installed. Once that is done, downloading the whole dataset, all three variants, is easy:
+
+```bash
+git clone https://huggingface.co/datasets/allenai/c4
+```
+
+If you want only one of the variants, you need some more commands:
+
+```bash
+git clone -n https://huggingface.co/datasets/allenai/c4
+cd c4
+git sparse-checkout init --cone
+git sparse-checkout set en
+```
+
+You can use `git sparse-checkout set` multiple times to select multiple datasets.