Update README.md

README.md

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 developed in the paper
 
 **Improving spectroscopic lithium abundances. Fitting functions for 3D non-LTE corrections in FGK stars of different metallicity**
-by A. Mott et al. (2020, Astronomy & Astrophysics 638, A58)
+by A. Mott et al. (2020, Astronomy & Astrophysics 638, A58, https://ui.adsabs.harvard.edu/link_gateway/2020A%26A...638A..58M/PUB_PDF)
 
 The three python functions are defined in `ff_all.py`, the IDL functions in `ff1.pro`, `ff2.pro`, `ff3.pro`.
 They are called in the example script `test_ff123.py` (`test_ff123.pro`) that produces the output file `test_ff123_py.out` (`test_ff123_idl.out`)