Add more docs for Feature Layer

not complete, but progress..

docs/features/custorders/purchasing/index.rst

@@ -0,0 +1,12 @@
+
+Purchasing
+==========
+
+In the context of Customer Orders, our concern here is making sure the
+Buyer knows about them and includes them in the Purchase Order(s)
+which they are already placing to the Vendor per normal procedure.
+
+TODO: this has been implemented a couple of different ways thus far
+but the "best" has yet to be decided...in particular (how) should the
+customer orders and purchase orders be linked?  it is a slight hassle
+up front but then can be leveraged during the receiving process...