add whatsnew entry about the new tutorial building machinery

doc/source/pre/whatsnew/1.5.rst

@@ -2,3 +2,13 @@ What's new in Kwant 1.5
 =======================
 
 This article explains the user-visible changes in Kwant 1.5.0.
+
+Improved tutorial building
+--------------------------
+Improving or adding to Kwant's tutorial is now much simpler. Now
+the text and code for each tutorial is kept in the same file, making
+it easy to see where changes need to be made, and images generated by
+the code are inserted directly into the document thanks to the magic of
+`jupyter-sphinx <https://github.com/jupyter-widgets/jupyter-sphinx/>`_.
+It has never been easier to get started contributing to Kwant by
+helping us improve our documentation.