Add reference paper

README.md

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 This is a XLM-roBERTa-base model trained on ~198M tweets and finetuned for sentiment analysis. The sentiment fine-tuning was done on 8 languages (Ar, En, Fr, De, Hi, It, Sp, Pt) but it can be used for more languages (see paper for details).
 
-- Paper: [XLM-T: A Multilingual Language Model Toolkit for Twitter](https://...). 
+- Paper: [XLM-T: A Multilingual Language Model Toolkit for Twitter](https://arxiv.org/abs/2104.12250). 
 - Git Repo: [XLM-T official repository](https://github.com/cardiffnlp/xlm-t).
 
 ## Example Pipeline