Add model card for BERTeus (#3649)

* Add model card for BERTeus

* Update README

model_cards/ixa-ehu/berteus-base-cased/README.md

+---
+language:
+- basque
+---
+
+# BERTeus base cased
+
+This is the Basque language pretrained model presented in [Give your Text Representation Models some Love: the Case for Basque](https://arxiv.org/pdf/2004.00033.pdf). This model has been trained on a Basque corpus comprising Basque crawled news articles from online newspapers and the Basque Wikipedia. The training corpus contains 224.6 million tokens, of which 35 million come from the Wikipedia.
+
+BERTeus has been tested on four different downstream tasks for Basque: part-of-speech (POS) tagging, named entity recognition (NER), sentiment analysis and topic classification; improving the state of the art for all tasks. See summary of results below:
+
+
+| Downstream task | BERTeus | mBERT | Previous SOTA |
+| --------------- | ------- | ------| ------------- |
+| Topic Classification	  | **76.77**   | 68.42 | 63.00 	    |
+| Sentiment    	  | **78.10**   | 71.02 | 74.02 	    |
+| POS   	  | **97.76**   | 96.37 | 96.10 	    |
+| NER    	  | **87.06**   | 81.52 | 76.72 	    |
+
+
+If using this model, please cite the following paper:
+```
+@inproceedings{agerri2020give,
+  title={Give your Text Representation Models some Love: the Case for Basque},
+  author={Rodrigo Agerri and I{\~n}aki San Vicente and Jon Ander Campos and Ander Barrena and Xabier Saralegi and Aitor Soroa and Eneko Agirre},
+  booktitle={Proceedings of the 12th International Conference on Language Resources and Evaluation},
+  year={2020}
+}
+```