Add a short documentation section about the io sub-package

docs/releases/upcoming/237.doc.rst

@@ -0,0 +1 @@
+Document the ``apptools.io`` and ``apptools.io.h5`` sub packages (#237)

docs/source/index.rst

@@ -10,6 +10,7 @@ AppTools Documentation
     undo/*
     selection/*
     naming/*
+    io/*
     api
 
 * :ref:`search`

docs/source/io/introduction.rst

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+File I/O
+========
+
+The :mod:`apptools.io` package provides a traited |File| object provides
+properties and methods for common file path manipulation operations.  Much of
+this functionality was implemented before Python 3 `pathlib`_ standard library
+became available to provide similar support.  For new code we encourage users
+to investigate if `pathlib`_ can satisfy their use cases before they turn to
+the `apptools.io` |File| object
+
+HDF5 File Support
+-----------------
+
+The :mod:`apptools.io.h5` sub-package provides a wrapper around `PyTables`_
+with a dictionary-style mapping.
+
+
+..
+   external links
+
+.. _pathlib: https://docs.python.org/3/library/pathlib.html
+.. _PyTables: https://www.pytables.org/
+
+
+..
+   # substitutions
+
+.. |File| replace:: :class:`~apptools.io.file.File`