Add link to arXiv.

README.md

@@ -15,3 +15,5 @@ RobeCzech model is released publicly at [LINDAT](https://hdl.handle.net/11234/1-
 
 Please cite the corresponding publication:
 - Milan Straka, Jakub Náplava, Jana Straková and David Samuel: Czech RoBERTa, a monolingual contextualized language representation model. Accepted to TSD 2021.
+
+Preprint of the paper is available at https://arxiv.org/abs/2105.11314.