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-sequence-nq/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 the RAG-Sequence 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.
+
+The model is a *uncased* model, which means that capital letters are simply converted to lower-case letters.
+
+The model consits of a *question_encoder*, *retriever* and a *generator*. The retriever extracts relevant passages from the *wiki_dpr* `train` datasets, which is linked above.
+The question_encoder and retriever are based on `facebook/dpr-question_encoder-single-nq-base` and `facebook/bart-large`, which were jointly finetuned on 
+on the *wiki_dpr* QA dataset in an end-to-end fashion.
+
+## Usage:
+
+**Note**: In the usage example below only the *dummy* retriever of *wiki_dpr* is used because the complete *lecagy* index requires over 75 GB of RAM.
+The model can generate answers to any factoid question as follows:
+
+```python
+from transformers import RagTokenizer, RagRetriever, RagSequenceForGeneration 
+ 
+tokenizer = RagTokenizer.from_pretrained("facebook/rag-sequence-nq") 
+retriever = RagRetriever.from_pretrained("facebook/rag-sequence-nq", index_name="exact", use_dummy_dataset=True) 
+model = RagSequenceForGeneration.from_pretrained("facebook/rag-sequence-nq", retriever=retriever) 
+ 
+input_dict = tokenizer.prepare_seq2seq_batch("how many countries are in europe", return_tensors="pt") 
+
+generated = model.generate(input_ids=input_dict["input_ids"]) 
+print(tokenizer.batch_decode(generated, skip_special_tokens=True)[0]) 
+
+# should give 54 => google says either 44 or 51
+```