Add ascl citation to docs

docs/source/citations.rst

@@ -2,6 +2,12 @@ NIFTy-related publications
 ==========================
 
 ::
+    @article{asclnifty5,
+      title={NIFTy5: Numerical Information Field Theory v5},
+      author={Arras, Philipp and Baltac, Mihai and Ensslin, Torsten A and Frank, Philipp and Hutschenreuter, Sebastian and Knollmueller, Jakob and Leike, Reimar and Newrzella, Max-Niklas and Platz, Lukas and Reinecke, Martin and others},
+      journal={Astrophysics Source Code Library},
+      year={2019}
+    }
 
     @software{nifty,
       author = {{Martin Reinecke, Theo Steininger, Marco Selig}},