Also update the README.md with the new citation information

README.md

@@ -58,6 +58,12 @@ physical model is something left to the users to explore.
 Acknowledgment & Citation
 -------------------------
 
+The SWIFT code was last described in this paper:
+https://ui.adsabs.harvard.edu/abs/2023arXiv230513380S.  The core solver, the
+numerical methods as well as many extensions where described there. We ask users
+running SWIFT for their research to please cite this paper when they present
+their results.
+
 In order to keep track of usage and measure the impact of the software, we
 kindly ask users publishing scientific results using SWIFT to add the following
 sentence to the acknowledgment section of their papers: