Create README.md (#4433)

model_cards/digitalepidemiologylab/covid-twitter-bert/README.md

+# COVID-Twitter-BERT (CT-BERT)
+BERT-large-uncased model, pretrained on a corpus of messages from Twitter about COVID-19
+
+## Overview
+This model was trained on 160M tweets collected between January 12 and April 16, 2020 containing at least one of the keywords "wuhan", "ncov", "coronavirus", "covid", or "sars-cov-2". These tweets were filtered and preprocessed to reach a final sample of 22.5M tweets (containing 40.7M sentences and 633M tokens) which were used for training.
+
+This model was evaluated based on downstream classification tasks, but it could be used for any other NLP task which can leverage contextual embeddings. 
+
+In order to achieve best results, make sure to use the same text preprocessing as we did for pretraining. This involves replacing user mentions, urls and emojis. You can find a script on our projects [GitHub repo](https://github.com/digitalepidemiologylab/covid-twitter-bert).
+
+## Example usage
+```python
+tokenizer = AutoTokenizer.from_pretrained("digitalepidemiologylab/covid-twitter-bert")
+model = TFAutoModel.from_pretrained("digitalepidemiologylab/covid-twitter-bert")
+```
+
+## References
+[1] Martin Müller, Marcel Salaté, Per E Kummervold. "COVID-Twitter-BERT: A Natural Language Processing Model to Analyse COVID-19 Content on Twitter" arXiv preprint arXiv:2005.07503 (2020).