Add a DOI to the default citation (#590)

There is no DOI link in the default citation, although there is one in the Bibtex. Adding it here makes it more likely for people to use it. 

This follows the format that Numpy uses, [here](https://numpy.org/citing-numpy/).

content/en/citing-scipy.md

@@ -16,7 +16,7 @@ Eric W. Moore, Jake VanderPlas, Denis Laxalde, Josef Perktold, Robert
 Cimrman, Ian Henriksen, E.A. Quintero, Charles R Harris, Anne M.
 Archibald, Antônio H. Ribeiro, Fabian Pedregosa, Paul van Mulbregt, and
 SciPy 1.0 Contributors. (2020) **SciPy 1.0: Fundamental Algorithms for
-Scientific Computing in Python**. _Nature Methods_, 17(3), 261-272.
+Scientific Computing in Python**. _Nature Methods_, 17(3), 261-272. DOI: [10.1038/s41592-019-0686-2](https://doi.org/10.1038/s41592-019-0686-2).
 
 Here\'s an example of a BibTeX entry: