Update README.md

README.md

 # HoloTomoToolbox
 
+## How to cite ?
+
+when using this toolbox, please cite: 
+
+A phase-retrieval toolbox for X-ray holography and tomography
+Leon M. Lohse, Anna-Lena Robisch, Mareike Töpperwien, Simon Maretzke, Martin Krenkel, Johannes Hagemann, and Tim Salditt
+Journal of Synchrotron Radiation 27, 852-859 (2020).
+https://doi.org/10.1107/S1600577520002398 
+
 ## Documentation
 
 See the [online-documentation](https://irp.pages.gwdg.de/holotomotoolbox/) and code examples in [examples/](examples/).