Add README

Author: sgugger

transformers_doc/README.md

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+# 🤗 Transformers doc notebooks
+
+These notebooks are automatically generated from the [🤗 Transformers documentation](https://huggingface.co/transformers/)
+so you should not make any direct modification here. If there is a typo to fix or a sentence to add, open a pull
+request in the [🤗 Transformers repo](https://github.com/huggingface/transformers) and fix the corresponding file in
+the `docs/source/` folder.
+
+If there is something that seems weirdly converted from the original doc file, open an issue in this repo and we will
+try to fix the conversion script.