More docs! w/ some emphasis on "reports" section

docs/data/reports/handler.rst

@@ -0,0 +1,19 @@
+
+==============
+Report Handler
+==============
+
+Each app will have exactly one "report handler" which is reponsible for the
+overall logic of running and emailing reports etc.  It also determines which
+report types are available to be ran.
+
+Rattail provides a default report handler so unless you have custom
+requirements, you will not need to define your own report handler.  See the
+:class:`~rattail:rattail.reporting.handlers.ReportHandler` class for more
+details.
+
+Programmatically you can access your report handler like so::
+
+   # assuming you already have config
+   app = config.get_app()
+   handler = app.get_report_handler()