add Stencila config files to docs

docs/source/config_files.rst

@@ -107,6 +107,22 @@ You also need to have a ``runtime.txt`` file that is formatted as
 ``r-<YYYY>-<MM>-<DD>``, where YYYY-MM-DD is a snapshot of MRAN that will be
 used for your R installation.
 
+``manifest.xml`` - Install Stencila
+===================================
+
+`Stencila <https://stenci.la/>`_ is an open source office suite for reproducible research.
+It is powered by the open file format `Dar <https://github.com/substance/dar>`_.
+
+If your repository contains a Stencila document, repo2docker detects it based on the file ``manifest.xml``.
+The required `execution contexts <https://stenci.la/learn/intro.html>` are extracted from a Dar article (i.e.
+files named ``*.jats.xml``).
+
+You may also have a ``runtime.txt`` and/or an ``install.R`` to manually configure the your R installation.
+
+To see example repositories, visit our
+`Stencila with R <https://github.com/binder-examples/stencila-r/>`_ and 
+`Stencila with Python <https://github.com/binder-examples/stencila-py>`_ examples.
+
 .. _postBuild:
 
 ``postBuild`` - Run code after installing the environment