Update README.md

Add benchmark paper to the README

README.md

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 **Lightshow** is a Python library for easily generating computational
-spectroscopy input files. If you use our code, please consider citing our [manuscript](https://doi.org/10.21105/joss.05182)
-in the Journal of Open Source Software.
+spectroscopy input files. If you use our code, please consider citing our [manuscript](https://doi.org/10.21105/joss.05182) in the Journal of Open Source Software and our multi-code benchmark [paper](https://doi.org/10.1103/PhysRevMaterials.8.013801), which provides the methods and default parameters used in Lightshow.
 
 Often, it can be daunting to create comprehensive, well documented
 databases of materials structures and their x-ray absorption spectra.