add "What's New" to the documentation

doc/source/index.rst

@@ -7,10 +7,15 @@ kwant documentation
 
    README file (includes installation instructions) <readme>
 
+.. toctree::
+   :maxdepth: 1
+
+   whatsnew/index
+
 .. toctree::
    :maxdepth: 2
 
-   tutorial/index.rst
+   tutorial/index
 
 .. toctree::
    :maxdepth: 2

doc/source/whatsnew/0.1.rst

+What's New in kwant 0.2
+=======================
+
+This article explains the user-visible changes in kwant 0.2.

doc/source/whatsnew/index.rst

+What's New in kwant
+===================
+
+.. toctree::
+    0.1