[ADD] inventory: What is a procurement rule?

https://docs.google.com/document/d/1eM7w-p83FHTZ7NrodIHCVWo3ocRfa1GrpBSvvoRB4Zw/edit#

inventory/routes/concepts/procurement_rule.rst

@@ -1,3 +1,77 @@
 ===========================
 What is a procurement rule?
-===========================
+===========================
+
+Overview
+========
+
+The procurement inventory control system begins with a customer's order.
+With this strategy, companies only make enough product to fulfill
+customer's orders. One advantage to the system is that there will be no
+excess of inventory that needs to be stored, thus reducing inventory
+levels and the cost of carrying and storing goods. However, one major
+disadvantage to the pull system is that it is highly possible to run
+into ordering dilemmas, such as a supplier not being able to get a
+shipment out on time. This leaves the company unable to fulfill the
+order and contributes to customer dissatisfaction.
+
+An example of a pull inventory control system is the make-to-order. The
+goal is to keep inventory levels to a minimum by only having enough
+inventory, not more or less, to meet customer demand. The MTO system
+eliminates waste by reducing the amount of storage space needed for
+inventory and the costs of storing goods.
+
+Configuration
+=============
+
+Procurement rules are part of the routes. Go to the Inventory
+application>Configuration>Settings and tick “Advance routing of products
+using rules”.
+
+.. image:: media/procurement_rule01.png
+    :align: center
+
+Procurement rules settings
+==========================
+
+The procurement rules are set on the routes. In the inventory
+application, go to Configuration > Routes.
+
+In the Procurement rules section, click on Add an item.
+
+.. image:: media/procurement_rule02.png
+    :align: center
+
+Here you can set the conditions of your rule. There are 3 types of
+action possibles :
+
+-   Move from another location rules
+
+-   Manufacturing rules that will trigger the creation of manufacturing
+    orders.
+
+-   Buy rules that will trigger the creation of purchase orders.
+
+.. note:: 
+    The Manufacturing application has to be installed in order to
+    trigger manufacturing rules.
+
+.. note:: 
+    The Purchase application has to be installed in order to trigger
+    **buy** rules.
+
+.. image:: media/procurement_rule03.png
+    :align: center
+
+.. demo:action:: stock.action_routes_form
+    
+    Try to create a procurement rule in our demo instance. 
+
+.. note:: 
+    Some Warehouse Configuration creates routes with procurement
+    rules already defined.
+
+.. seealso::
+    * :doc:`push_rule`
+    * :doc:`inter_warehouse`
+    * :doc:`cross_dock`