Docs: Explain docs/ directory structure

CONTRIBUTING.rst

     cd docs
     ./test_docs.sh
 
+The actual docs are in ``/docs/source``. This directory structure is a
+simplified version of what Django does.
+
+* ``internals/`` contains everything related to Oscar itself, e.g. contributing
+  guidelines or design philosophies.
+* ``ref/`` is the reference documentation, esp. consisting of
+* ``ref/apps/`` which should be a guide to each Oscar core app, explaining it's
+  function, the main models, how it relates to the other apps, etc.
+* ``topics/`` will contain "meta" articles, explaining how things tie together
+  over several apps, or how Oscar can be combined with other solutions.
+* ``howto/`` contains tutorial-style descriptions on how to solve a certain
+  problem. Ideally, those will be kept to a minimum; as the other docs should
+  be understandable enough that necessary steps to achieve a certain goal are
+  obvious.
+* ``_old/`` contains legacy documents that have not been moved into the new
+  structure. Please help get rid of those!
+
+``/index.rst`` is designed as the entry point, and diverges from above
+structure to make the documentation more approachable. Other ``index.rst``
+files should only be created if there's too many files to list them all.
+E.g. ``/index.rst`` directly links to all files in ``topics/`` and
+``internals/``, but there's an ``index.rst`` both for the files in ``howto/``
+and ``ref/apps/``.
+
+
 Conventions
 ===========