Add some basic docs for data import/export

docs/data/importing/concepts.rst

@@ -0,0 +1,37 @@
+
+Concepts
+========
+
+.. _import-vs-export:
+
+Import vs. Export
+-----------------
+
+From the framework's perspective, everything is actually an "import"
+and never an "export" per se, i.e. data always flows from a source and
+to a destination - or in other words, the "local" side always
+*imports* from the "host" side.  But see :doc:`exporting` for more
+about that.
+
+.. _importer-vs-import-handler:
+
+Importer vs. Import Handler
+---------------------------
+
+All import (and export) tasks are accomplished by way of 2 things:
+
+The "importer" is responsible for reading data from one place and
+writing it to another place.  But it cannot run itself, it just
+contains the e.g. table-specific read/write logic.
+
+The "import handler" is responsible for running one or more importers.
+It also is (usually) responsible for "transaction-level" logic, e.g.
+connecting to a database, and ultimately committing that transaction.
+It usually has no table-specific logic, as that is the importer's job.
+
+For instance you might want to import all customers and products from
+your POS into Rattail; this would require something like:
+
+ * import handler for POS -> Rattail
+ * importer for Customer data
+ * importer for Product data