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/huawei-noah/DynaBERT_SST-2/README.md

README.md

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+## DynaBERT: Dynamic BERT with Adaptive Width and Depth
+
+* DynaBERT can flexibly adjust the size and latency by selecting adaptive width and depth, and 
+the subnetworks of it have competitive performances as other similar-sized compressed models.
+The training process of DynaBERT includes first training a width-adaptive BERT and then 
+allowing both adaptive width and depth using knowledge distillation. 
+
+* This code is modified based on the repository developed by Hugging Face: [Transformers v2.1.1](https://github.com/huggingface/transformers/tree/v2.1.1), and is released in [GitHub](https://github.com/huawei-noah/Pretrained-Language-Model/tree/master/DynaBERT).
+
+### Reference
+Lu Hou, Zhiqi Huang, Lifeng Shang, Xin Jiang, Xiao Chen, Qun Liu.
+[DynaBERT: Dynamic BERT with Adaptive Width and Depth](https://arxiv.org/abs/2004.04037).
+```
+@inproceedings{hou2020dynabert,
+  title = {DynaBERT: Dynamic BERT with Adaptive Width and Depth},
+  author = {Lu Hou, Zhiqi Huang, Lifeng Shang, Xin Jiang, Xiao Chen, Qun Liu},  
+  booktitle = {Advances in Neural Information Processing Systems},
+  year = {2020}
+}
+```