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/mrm8488/spanbert-large-finetuned-tacred/README.md

README.md

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+---
+language: en
+thumbnail:
+---
+
+# SpanBERT large fine-tuned on TACRED
+
+[SpanBERT](https://github.com/facebookresearch/SpanBERT) created by [Facebook Research](https://github.com/facebookresearch) and fine-tuned on [TACRED](https://nlp.stanford.edu/projects/tacred/) dataset by [them](https://github.com/facebookresearch/SpanBERT#finetuned-models-squad-1120-relation-extraction-coreference-resolution)
+
+## Details of SpanBERT
+
+[SpanBERT: Improving Pre-training by Representing and Predicting Spans](https://arxiv.org/abs/1907.10529)
+
+## Dataset 📚
+
+[TACRED](https://nlp.stanford.edu/projects/tacred/) A large-scale relation extraction dataset with 106k+ examples over 42 TAC KBP relation types.
+
+## Model fine-tuning 🏋️‍
+
+You can get the fine-tuning script [here](https://github.com/facebookresearch/SpanBERT)
+
+```bash
+python code/run_tacred.py \
+  --do_train \
+  --do_eval \
+  --data_dir <TACRED_DATA_DIR> \
+  --model spanbert-large-cased \
+  --train_batch_size 32 \
+  --eval_batch_size 32 \
+  --learning_rate 2e-5 \
+  --num_train_epochs 10 \
+  --max_seq_length 128 \
+  --output_dir tacred_dir \
+  --fp16
+```
+
+## Results Comparison 📝
+
+|                   | SQuAD 1.1     | SQuAD 2.0  | Coref   | TACRED |
+| ----------------------  | ------------- | ---------  | ------- | ------ |
+|                         | F1            | F1         | avg. F1 |  F1    |
+| BERT (base)             | 88.5*         | 76.5*      | 73.1    |  67.7  |
+| SpanBERT (base)         | [92.4*](https://huggingface.co/mrm8488/spanbert-base-finetuned-squadv1)         | [83.6*](https://huggingface.co/mrm8488/spanbert-base-finetuned-squadv2)      | 77.4    |  [68.2](https://huggingface.co/mrm8488/spanbert-base-finetuned-tacred)  |
+| BERT (large)            | 91.3          | 83.3       | 77.1    |  66.4  |
+| SpanBERT (large)        | [94.6](https://huggingface.co/mrm8488/spanbert-large-finetuned-squadv1)        | [88.7](https://huggingface.co/mrm8488/spanbert-large-finetuned-squadv2)     | 79.6    |  **70.8** (this one)  |
+
+
+Note: The numbers marked as * are evaluated on the development sets because those models were not submitted to the official SQuAD leaderboard. All the other numbers are test numbers.
+
+
+> Created by [Manuel Romero/@mrm8488](https://twitter.com/mrm8488)
+
+> Made with <span style="color: #e25555;">&hearts;</span> in Spain