[ADD] inventory: What is a push rule?

https://docs.google.com/document/d/1um_8Fpk3RVT6pm62WdFXycZZWPmXzLVL6TaIbwaPxpE/edit#

inventory/routes/concepts/push_rule.rst

@@ -1,3 +1,85 @@
 ====================
 What is a push rule?
-====================
+====================
+
+Overview 
+=========
+
+The push system of inventory control involves forecasting inventory
+needs to meet customer demand. Companies must predict which products
+customers will purchase along with determining what quantity of goods
+will be purchased. The company will in turn produce enough product to
+meet the forecast demand and sell, or push, the goods to the consumer.
+Disadvantages of the push inventory control system are that forecasts
+are often inaccurate as sales can be unpredictable and vary from one
+year to the next. Another problem with push inventory control systems is
+that if too much product is left in inventory. This increases the
+company's costs for storing these goods. An advantage to the push system
+is that the company is fairly assured it will have enough product on
+hand to complete customer orders, preventing the inability to meet
+customer demand for the product.
+
+A push flow indicates how locations are chained with the other ones. As
+soon as a given quantity of products is moved in the source location, a
+chained move is automatically foreseen according to the parameters set
+on the flow specification (destination location, delay, type of move,
+journal). It can be triggered automatically or manually.
+
+Configuration
+=============
+
+Push rules are part of the routes. Go to the menu 
+:menuselection:`Inventory --> Configuration --> Settings` 
+and tick **Advance routing of products using rules**.
+
+.. image:: media/push_rule01.png
+    :align: center
+
+Push rules settings
+===================
+
+The push rules are set on the routes. 
+Go to :menuselection:`Configuration --> Routes`.
+
+In the push rule section, click on **Add an item**.
+
+.. image:: media/push_rule02.png
+    :align: center
+
+Here you can set the conditions of your rule. In this example, when a
+good is in **Input location**, it needs to be moved to the quality control.
+In the 3 steps receipts, another push rule will make the goods that are
+in the quality control location move to the stock.
+
+.. image:: media/push_rule03.png
+    :align: center
+
+.. demo:action:: stock.action_routes_form
+    
+    Try to create a push rule in our demo instance. 
+
+.. note::
+    Some warehouse configuration creates routes with push rules
+    already defined.
+
+Stock transfers
+===============
+
+The push rule will trigger stock transfer. According to the rule set on
+your route, you will see that some transfers might be ready and other
+are waiting.
+
+The push rule that was set above will create moves from **WH/Input**
+location to the **WH/Quality Control** location.
+
+.. image:: media/push_rule04.png
+    :align: center
+
+In this example, another move is waiting according to the second push
+rule, it defines that when the quality control is done, the goods will
+be moved to the main stock.
+
+.. seealso::
+    * :doc:`procurement_rule`
+    * :doc:`inter_warehouse`
+    * :doc:`cross_dock`