add kwant.contrib to docs

doc/source/contrib/digest.rst

+:mod:`kwant.contrib.digest` -- Random-access random numbers
+===========================================================
+
+.. automodule:: kwant.contrib.digest
+   :members:

doc/source/contrib/index.rst

+:mod:`kwant.contrib` -- 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`.
+
+For simplicity, all contributions share the same license as kwant.  (This
+requirement may be relaxed to allow other free licenses if requested.)
+
+.. toctree::
+   :maxdepth: 1
+
+   digest
+   rmt

doc/source/contrib/rmt.rst

+:mod:`kwant.contrib.rmt` -- RMT (random matrix theory) Hamiltonians
+===================================================================
+
+.. automodule:: kwant.contrib.rmt
+   :members:

doc/source/index.rst

@@ -34,4 +34,9 @@ have made it possible <authors>`.
 
    reference/index
 
+.. toctree::
+   :maxdepth: 2
+
+   contrib/index
+
 * :ref:`genindex`

doc/source/whatsnew/0.3.rst

@@ -156,3 +156,8 @@ Modes or self-energies can now be precomputed before passing the system to a
 solver, using the method `~kwant.system.FiniteSystem.precalculate`. This may
 save time, when the linear system has to be solved many times with the same
 lead parameters.
+
+Inclusion of contributed modules
+--------------------------------
+kwant now contains a sub-package :mod:`kwant.contrib` that contains various
+modules that are useful with kwant but are not considered to be part of it.

kwant/contrib/digest.py

@@ -6,7 +6,7 @@
 # the AUTHORS file at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-"""A module for "functional" random numbers.
+"""Random-access random numbers
 
 This module provides routines that given some input compute a "random" output
 that depends on the input in a (cryptographically) intractable way.