Migrate model card from transformers-repo

Read announcement at https://discuss.huggingface.co/t/announcement-all-model-cards-will-be-migrated-to-hf-co-model-repos/2755
Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/facebook/rag-token-base/README.md

README.md

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+---
+language: en
+license: apache-2.0
+datasets:
+- wiki_dpr
+thumbnail: https://huggingface.co/front/thumbnails/facebook.png
+---
+## RAG
+
+This is a non-finetuned version of the RAG-Token model of the the paper [Retrieval-Augmented Generation for Knowledge-Intensive NLP Tasks](https://arxiv.org/pdf/2005.11401.pdf) 
+by Patrick Lewis, Ethan Perez, Aleksandara Piktus et al.
+
+Rag consits of a *question encoder*, *retriever* and a *generator*. The retriever should be a `RagRetriever` instance. The *question encoder* can be any model that can be loaded with `AutoModel` and the *generator* can be any model that can be loaded with `AutoModelForSeq2SeqLM`. 
+
+This model is a non-finetuned RAG-Token model and was created as follows:
+
+```python
+from transformers import RagTokenizer, RagRetriever, RagTokenForGeneration, AutoTokenizer
+
+model = RagTokenForGeneration.from_pretrained_question_encoder_generator("facebook/dpr-question_encoder-single-nq-base", "facebook/bart-large")
+
+question_encoder_tokenizer = AutoTokenizer.from_pretrained("facebook/dpr-question_encoder-single-nq-base")
+generator_tokenizer = AutoTokenizer.from_pretrained("facebook/bart-large")
+
+tokenizer = RagTokenizer(question_encoder_tokenizer, generator_tokenizer)
+model.config.use_dummy_dataset = True
+model.config.index_name = "exact"
+retriever = RagRetriever(model.config, question_encoder_tokenizer, generator_tokenizer)
+
+model.save_pretrained("./")
+tokenizer.save_pretrained("./")
+retriever.save_pretrained("./")
+```
+
+Note that the model is *uncased* so that all capital input letters are converted to lower-case.
+
+## Usage:
+
+*Note*: the model uses the *dummy* retriever as a default. Better results are obtained by using the full retriever, 
+by setting `config.index_name="legacy"` and `config.use_dummy_dataset=False`.
+The model can be fine-tuned as follows:
+
+```python
+from transformers import RagTokenizer, RagRetriever, RagTokenForGeneration
+
+tokenizer = RagTokenizer.from_pretrained("facebook/rag-token-base")
+retriever = RagRetriever.from_pretrained("facebook/rag-token-base")
+model = RagTokenForGeneration.from_pretrained("facebook/rag-token-base", retriever=retriever)
+
+input_dict = tokenizer.prepare_seq2seq_batch("who holds the record in 100m freestyle", "michael phelps", return_tensors="pt") 
+
+outputs = model(input_dict["input_ids"], labels=input_dict["labels"])
+
+loss = outputs.loss
+
+# train on loss
+```