docs: add basic docs for batch feature

docs/narr/batch/handlers.rst

@@ -0,0 +1,38 @@
+
+Batch Handlers
+==============
+
+The :term:`batch handler` is responsible for all logic surrounding a
+:term:`batch`: creation, updates, execution.
+
+Each batch handler is associated with one :term:`batch type`.  There
+may be more than one handler defined for a given batch type.  However
+only one handler will be designated as the "default" for each batch
+type (based on config and app defaults).
+
+
+Class Definition
+----------------
+
+A new batch handler class should inherit from
+:class:`~wuttjamaican.batch.BatchHandler`, which provides most of the
+typical housekeeping logic etc.
+
+Note that it also must declare the
+:attr:`~wuttjamaican.batch.BatchHandler.model_class`, which ultimately
+determines the handler's :term:`batch type`.
+
+You will need to define/override some methods for the new handler to
+be useful::
+
+   from wuttjamaican.batch import BatchHandler
+   from poser.db.model import InventoryBatch
+
+   class InventoryBatchHandler(BatchHandler):
+       """ Handler for inventory count batches """
+
+       # direct reference to batch model class
+       model_class = InventoryBatch
+
+       def execute(self, batch, user=None, progress=None, **kwargs):
+           """ export batch row data to CSV """