docs: add some narrative docs to explain basic concepts

still needs a lot of work i'm sure..gotta start somewhere

docs/narr/concepts.rst

@@ -0,0 +1,54 @@
+
+Concepts
+========
+
+Things hopefully are straightforward but it's important to get the
+following straight in your head; the rest will come easier if you do.
+
+
+Source vs. Target
+-----------------
+
+Data always flows from source to target, it is the #1 rule.
+
+Docs and command output will always reflect this, e.g. **CSV →
+Wutta**.
+
+Source and target can be anything as long as the :term:`import
+handler` and :term:`importer(s) <importer>` implement the desired
+logic.  The :term:`app database` is often involved but not always.
+
+
+Import vs. Export
+-----------------
+
+Surprise, there is no difference.  After all from target's perspective
+everything is really an import.
+
+Sometimes it's more helpful to think of it as an export, e.g. **Wutta
+→ CSV** really seems like an export.  In such cases the
+:attr:`~wuttasync.importing.handlers.ImportHandler.orientation` may be
+set to reflect the distinction.
+
+
+.. _import-handler-vs-importer:
+
+Import Handler vs. Importer
+---------------------------
+
+The :term:`import handler` is sort of the "wrapper" around one or more
+:term:`importers <importer>` and the latter contain the table-specific
+sync logic.
+
+In a DB or similar context, the import handler will make the
+connection, then invoke all requested importers, then commit
+transaction at the end (or rollback if dry-run).
+
+And each importer will read data from source, and usually also read
+data from target, then compare data sets and finally write data to
+target as needed.  But each would usually do this for just one table.
+
+See also the base classes for each:
+
+* :class:`~wuttasync.importing.handlers.ImportHandler`
+* :class:`~wuttasync.importing.base.Importer`