Add some basic docs for data import/export

docs/data/importing/exporting.rst

@@ -0,0 +1,33 @@
+
+Data Exporters
+==============
+
+Within the framework there is no such thing as an "export" per se.
+Everything functions as an import, i.e. the "local" side *imports*
+from the "host" side.  See :ref:`import-vs-export` for more on that.
+
+But there are a couple of caveats:
+
+
+.. _declare-exporter:
+
+Declaring an Exporter
+---------------------
+
+In practice it's possible to mark any importer as being of the
+"export" persuasion.  This doesn't (usually) affect the underlying
+logic, but is done e.g. to make command line usage and output more
+sensible for us humans.
+
+To do this, set the
+:attr:`~rattail:rattail.importing.handlers.ImportHandler.direction`
+for your import handler to the value, ``'export'``.
+
+
+.. _common-exporters:
+
+"Common" Exporters
+------------------
+
+TODO: describe e.g. SQLAlchemy -> CSV export, which dynamically
+generates its own importer classes