Update README.md

Add citation info.

README.md

@@ -136,7 +136,7 @@ TBA
 
 ## Citation
 
-*Cite the source where this metric was introduced.*
+This metric was developed as part of an IEA R&D project, [Improving Parental Occupation Coding Procedures with AI](https://www.iea.nl/sites/default/files/2024-09/Improving-Parental-Occupation-Coding-Procedures-AI.pdf) (Duckworth et al, 2024). The metric is used to evaluate the multilingual [ICILS XLM-R ISCO](https://huggingface.co/ICILS/xlm-r-icils-ilo) classification model.
 
 TBA