Update README.md

Add license and citation

README.md

@@ -2,10 +2,27 @@
 language:
 - en
 pipeline_tag: token-classification
+license: apache-2.0
 ---
 
 Named Entity Recognition (NER) model to recognize chemical entities. 
 
+Please cite our work:
+
+```
+@article{NILNKER2022,
+  title = {NILINKER: Attention-based approach to NIL Entity Linking},
+  journal = {Journal of Biomedical Informatics},
+  volume = {132},
+  pages = {104137},
+  year = {2022},
+  issn = {1532-0464},
+  doi = {https://doi.org/10.1016/j.jbi.2022.104137},
+  url = {https://www.sciencedirect.com/science/article/pii/S1532046422001526},
+  author = {Pedro Ruas and Francisco M. Couto},
+}
+```
+
 [PubMedBERT](https://huggingface.co/microsoft/BiomedNLP-PubMedBERT-base-uncased-abstract-fulltext) fine-tuned on the following datasets:
 
 - [Chemdner patents CEMP corpus](https://biocreative.bioinformatics.udel.edu/resources/corpora/chemdner-patents-cemp-corpus/) (train, dev, test sets)