Updated README

README.md

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 The `ikpls` software package provides fast and efficient tools for PLS (Partial Least Squares) modeling. This package is designed to help researchers and practitioners handle PLS modeling faster than previously possible - particularly on large datasets.
 
+## Citation
+If you use the `ikpls` software package for your work, please cite [this Journal of Open Source Software paper](https://joss.theoj.org/papers/10.21105/joss.06533). If you use the fast cross-validation algorithm implemented in `ikpls.fast_cross_validation.numpy_ikpls`, please also cite [this arXiv preprint](https://arxiv.org/abs/2401.13185).
+
 ## Unlock the Power of Fast and Stable Partial Least Squares Modeling with IKPLS
 
 Dive into cutting-edge Python implementations of the IKPLS (Improved Kernel Partial Least Squares) Algorithms #1 and #2 [[1]](#references) for CPUs, GPUs, and TPUs. IKPLS is both fast [[2]](#references) and numerically stable [[3]](#references) making it optimal for PLS modeling.