Includes paper reference in Sphinx documentation.

doc/index.rst

@@ -9,7 +9,7 @@ GalMag Documentation
 This is the documentation of `GalMag <https://github.com/luizfelippesr/galmag>`_ is a `Python <http://www.python.org>`_  package for computing galactic magnetic
 fields based on mean field dynamo theory.
 
-For a detailed description of the physics, please refer to the code paper.
+For a detailed description of the physics, please refer to the code `paper <https://ui.adsabs.harvard.edu/abs/2019A&A...623A.113S/abstract>`_.
 
 A quick tour through the functionality and a description of the features and usage of the package can by found in the
 `tutorial <http://nbviewer.jupyter.org/url/www.mas.ncl.ac.uk/~nlfsr/galmag/galmag_tutorial.ipynb>`_