[ADD] inventory: How is the scheduled delivery date computed?

https://docs.google.com/document/d/1Lpbb6G5eOeCtEXcxXOFVdnueDmPQYo5Fo3SEdCCHOPg/edit

inventory/delivery.rst

@@ -9,4 +9,5 @@ Delivery Orders
    delivery/inventory_flow
    delivery/one_step
    delivery/two_steps
-   delivery/three_steps
+   delivery/three_steps
+   delivery/scheduled_dates

inventory/delivery/scheduled_dates.rst

@@ -0,0 +1,154 @@
+============================================
+How is the scheduled delivery date computed?
+============================================
+
+Scheduled dates are computed in order to be able to plan deliveries,
+receptions and so on. Depending on the habits of your company Odoo
+automatically generates scheduled dates via the scheduler. The Odoo
+scheduler computes everything per line, whether it's a manufacturing
+order, a delivery order, a sale order, etc. The dates that are computed
+are dependent on the different leads times configured in Odoo.
+
+Configuring lead times
+======================
+
+Configuring **lead times** is a first essentiel move in order to compute
+scheduled dates. Lead times are the delays (in term of delivery,
+manufacturing, ...) promised to your different partners and/or clients.
+
+Configuration of the different lead times are made as follows:
+
+At a product level
+------------------
+
+-  **Supplier lead time**:
+
+Is the time needed for the supplier to deliver your purchased product.
+To configure the supplier lead time select a product, and go in the
+**Inventory** tab. You will have to add a vendor to your product in order
+to select a supplier lead time.
+
+.. image:: media/scheduled_dates05.png
+   :align: center
+
+.. tip:: 
+    Do not forget that it is possible to add different vendors and thus
+    different delivery lead times depending on the vendor.
+
+Once a vendor is selected, just open its form and fill its **Delivery lead
+time**. In this case security days have no influence, the scheduled
+delivery days will be equal to: **Date** of the purchase order + **Delivery
+Lead Time**.
+
+.. image:: media/scheduled_dates02.png
+   :align: center
+
+-  **Customer lead time**:
+
+Customer lead time is the time needed to get your product from your
+store / warehouse to your customer. It can be configured for any
+product. Simply select a product, go into the sales tab and indicate
+your **Customer lead time**.
+
+.. image:: media/scheduled_dates01.png
+   :align: center
+
+-  **Manufacturing lead time**:
+
+At the same page it is possible to configure the **Manufacturing Lead
+Time** as well. Manufacturing lead time is the time needed to
+manufacture the product.
+
+.. tip::
+    Don’t forget to tick the manufacturing box in inventory if you
+    want to create manufacturing routes.
+
+At the company level
+--------------------
+
+At company level, it is possible to configure **security days** in order
+to cope with eventual delays and to be sure to meet your engagements.
+The idea is to subtract **backup** days from the **computed scheduled date**
+in case of delays.
+
+-  **Sales Safety days**:
+
+Sales safety days are **back-up** days to ensure you will be able to
+deliver your clients engagements in times. They are margins of errors
+for delivery lead times. Security days are the same logic as the early
+wristwatch, in order to arrive on time. The idea is to subtract the
+numbers of security days from the calculation and thus to compute a
+scheduled date earlier than the one you promised to your client. In that
+way you are sure to be able to keep your commitment.
+
+To set ut your security dates, go to :menuselection:`Settings --> General settings` and
+click on **Configure your company data**.
+
+.. image:: media/scheduled_dates04.png
+   :align: center
+
+Once the menu is open, go in the configuration tab and indicate the
+number of safety days.
+
+-  **Purchase Safety days**:
+
+Purchase days follow to the same logic than sales security days.
+
+They are margins of error for vendor lead times. When the system
+generates purchase orders for procuring products, they will be scheduled
+that many days earlier to cope with unexpected vendor delays. Purchase
+lead time can be found in the same menu as the sales safety days
+
+.. image:: media/scheduled_dates03.png
+   :align: center
+
+.. tip::
+    Note that you can also configure a default 
+    Manufacturing lead time from here.
+
+At route level
+--------------
+
+The internal transfers that a product might do due to the movement of
+stocks can also influence the computed date.
+
+The delays due to internal transfers can be specified in the **inventory**
+app when creating a new push rule in a route.
+
+Go to the push rules section on a route form to set a delay.
+
+.. image:: media/scheduled_dates06.png
+   :align: center
+
+At sale order level:
+--------------------
+
+-  **Requested date**:
+
+Odoo offers the possibility to select a requested date by the client by
+indicating the date in the other information tab of the sales order.
+If this date is earlier than the theoreticaly computed date odoo will
+automatically display a warning.
+
+.. image:: media/scheduled_dates07.png
+   :align: center
+
+Example
+=======
+
+As an example, you may sell a car today (January 1st), that is purchased
+on order, and you promise to deliver your customer within 20 days
+(January 20). In such a scenario, the scheduler may trigger the
+following events, based on your configuration:
+
+-  January 19: actual scheduled delivery (1 day of Sales Safety days)
+
+-  January 18: receive the product from your supplier (1 day of Purchase
+   days)
+
+-  January 10: deadline to order at your supplier (9 days of supplier
+   delivery lead time)
+
+-  January 8: trigger a purchase request to your purchase team, since
+   the team need on average 2 days to find the right supplier and
+   order.