tutorial: mention the paper

doc/source/tutorial/introduction.rst

@@ -7,6 +7,15 @@ commented extensively. In addition, you will find notes about more subtle,
 technical details at the end of each example. At first reading, these notes may
 be safely skipped.
 
+A scientific article about Kwant is available as well:
+
+TO-BE-FILLED-IN
+
+The article introduces Kwant with a somewhat different focus than the tutorial
+and it is the authors' intention that both texts complement each other.  While
+the tutorial is more "hands-on", the article presents Kwant in a more conceptual
+way, as well as discussing questions of design and performance.
+
 Quantum transport
 .................