Update Usefult-link.md

add citation information

docs/Reference-guide/Usefult-link.md

@@ -20,3 +20,6 @@ We'd love to hear from you!
 1. To provide feedback or ask a question, contact the GWAS Catalog team on [email protected].
 2. If you believe you’ve encountered a bug, please don’t hesitate to report it in our [GitHub repository](https://github.com/EBISPOT/gwas-sumstats-harmoniser/issues/new?assignees=&labels=bug&projects=&template=bug_report.yml&title=%5BBug%5D%3A+).
 3. If you encounter any issues while running the pipeline, feel free to join the discussion in the [gwas-sumstats-harmoniser Discussions](https://github.com/EBISPOT/gwas-sumstats-harmoniser/discussions)!
+
+## Citation:
+If you use the NHGRI-EBI GWAS Catalog tool in your research,  please refer to the "[How to Cite the NHGRI-EBI GWAS Catalog, Data, or Diagrams](https://www.ebi.ac.uk/gwas/docs/about#:~:text=How%20to%20cite%20the%20NHGRI%2DEBI%20GWAS%20Catalog%2C%20data%20therein%20or%20diagram%3A)" section on our website for proper citation guidelines.