add a suggested sentence for citing kwant

content/citing.txt

@@ -4,8 +4,12 @@ Citing Kwant
 We provide Kwant as free software under a `BSD license </license.html>`_ as a
 service to the physics community.  If you have used Kwant for work that has
 lead to a scientific publication, please mention the fact that you used it
-explicitly in the text body.  In addition, we ask you to cite the main paper
-that introduces Kwant:
+explicitly in the text body.  For example you may add:
+
+  *The numerical calculations were performed using the Kwant code.*
+
+to the description of your numerical calculations.  In addition, we ask you to
+cite the main paper that introduces Kwant:
 
   C. W. Groth, M. Wimmer, A. R. Akhmerov, X. Waintal, *Kwant: a software package
   for quantum transport*, `arXiv:1309.2926 <http://arxiv.org/abs/1309.2926>`_.