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/gpt2-medium-README.md

README.md

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+Test the full generation capabilities here: https://transformer.huggingface.co/doc/gpt2-large