remove kwant.contrib from title, minor text changes

doc/source/contrib/index.rst

-:mod:`kwant.contrib` -- Contributed modules
-===========================================
+Contributed modules
+===================
 
 .. module:: kwant.contrib
 
-Although this collection of modules is available as part of the official kwant
-distribution, it is not considered to be an integral part of kwant.  The
-intention is to keep interesting modules that are useful with kwant here (maybe
-even experimental ones) to give users an easier access to them, and to give them
-wider exposure, so that they can be improved faster.  Users of kwant are welcome
-to submit their own modules for inclusion in :mod:`kwant.contrib`.
+:mod:`kwant.contrib` is a collection of independent modules that is part of the
+official kwant distribution, but is not considered to be an integral part of
+kwant.  The intention is to keep interesting modules that are useful with kwant
+here (maybe even experimental ones) to give users an easier access to them, and
+them wider exposure.  Users of kwant are welcome to submit their own modules for
+inclusion.
 
 For simplicity, all contributions share the same license as kwant.  (This
 requirement may be relaxed to allow other free licenses if requested.)