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/Geotrend/bert-base-bg-cased/README.md

README.md

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+---
+language: bg
+
+datasets: wikipedia
+
+license: apache-2.0
+---
+
+# bert-base-bg-cased
+
+We are sharing smaller versions of [bert-base-multilingual-cased](https://huggingface.co/bert-base-multilingual-cased) that handle a custom number of languages.
+
+Unlike [distilbert-base-multilingual-cased](https://huggingface.co/distilbert-base-multilingual-cased), our versions give exactly the same representations produced by the original model which preserves the original accuracy.
+
+For more information please visit our paper: [Load What You Need: Smaller Versions of Multilingual BERT](https://www.aclweb.org/anthology/2020.sustainlp-1.16.pdf).
+
+## How to use
+
+```python
+from transformers import AutoTokenizer, AutoModel
+
+tokenizer = AutoTokenizer.from_pretrained("Geotrend/bert-base-bg-cased")
+model = AutoModel.from_pretrained("Geotrend/bert-base-bg-cased")
+
+```
+
+To generate other smaller versions of multilingual transformers please visit [our Github repo](https://github.com/Geotrend-research/smaller-transformers).
+
+### How to cite
+
+```bibtex
+@inproceedings{smallermbert,
+  title={Load What You Need: Smaller Versions of Mutlilingual BERT},
+  author={Abdaoui, Amine and Pradel, Camille and Sigel, Grégoire},
+  booktitle={SustaiNLP / EMNLP},
+  year={2020}
+}
+```
+
+## Contact 
+
+Please contact amine@geotrend.fr for any question, feedback or request.