Add more docs for Feature Layer

not complete, but progress..

docs/features/purchasing/ordering/additems.rst

@@ -0,0 +1,22 @@
+
+Adding Items
+============
+
+When creating a new Purchase Order using Rattail, the user first makes
+a new Ordering batch, then adds items to it.
+
+There are three main "workflows" for adding the items.
+
+The first is where the user sees the (at first empty) batch and then
+does some sort of product lookup to identify an item to add.  The
+lookup/add process is repeated as needed.
+
+The other way is what Rattail calls the "worksheet view" - but it's
+only suitable for vendors with relatively small product lines.  In
+this view the user is presented with a "worksheet" form.  All items
+available from the vendor are visible on the worksheet, and the user
+enters desired quantities for applicable line items.
+
+There also is a mobile interface for ordering, which if used on a
+smart phone with bluetooth scanner attached, can be used for
+"in-aisle" ordering by scanning shelf tags.