[ADD] purchase: How to trigger the purchase of products based on sales?

https://docs.google.com/document/d/1cEAZdC-9hANhvTH5tWNm9Uys6ccpfvOgjKXlYby5vpw/edit?usp=sharing

purchase/replenishment/flows/purchase_triggering.rst

@@ -2,4 +2,112 @@
 How to trigger the purchase of products based on sales?
 =======================================================
 
-*Coming soon...*
+Overview
+========
+
+When you work in just-in-time, you don't manage stock so you directly
+order the product you need from your vendor.
+
+The usual flow is:
+
+1. Create a sale order
+
+2. Purchase the product
+
+3. Receive and pay the bill
+
+4. Deliver your product
+
+5. Invoice your customer
+
+Product configuration
+=====================
+
+In the purchases application, open the **Purchase** menu and click on
+**Products**. Open the product on which you want to do your purchases based
+on sales.
+
+Next to Routes, tick **Buy** and **Make to order** as a procurement method.
+When you are generating sales order, Odoo will automatically reorder the
+same quantity through procurement.
+
+.. image:: media/purchase_triggering01.png
+    :align: center
+
+Don't forget to set a vendor otherwise the rule won't be triggered.
+
+You can also configure minimum stock rules that will trigger the
+purchase orders.
+
+.. note::
+    To know how to configure a minimum stock rule, please read the document 
+    :doc:`setup_stock_rule`.
+
+Process
+=======
+
+Sale order
+----------
+
+To create a sale order, go to the **Sales** application, 
+:menuselection:`Sales --> Sales order` and create a new sale order.
+
+.. image:: media/purchase_triggering02.png
+    :align: center
+
+After confirming it, you will see one **Delivery** associated with this
+sale order on the **button** on the top of it.
+
+.. image:: media/purchase_triggering03.png
+    :align: center
+
+Click on the **Delivery** button to see the transfer order. The status
+of the outgoing shipment is **Waiting Another Operation**. It won't be done
+until the purchase order is confirmed and received.
+
+Purchase order
+--------------
+
+The purchase order is automatically created. Go to the **Purchase**
+application :menuselection:`Purchase --> Request for Quotation`. 
+The source document is the sale order that triggered the procurement.
+
+.. image:: media/purchase_triggering04.png
+    :align: center
+
+.. note::
+    If you make some more sales that trigger procurements to the same
+    vendor, it will be added to the existing request for quotation. Once it
+    is confirmed, the next procurements will create a new request for
+    quotation.
+
+Receipt and delivery
+--------------------
+
+Go the the **Inventory** application. Click on **# To Receive** on the
+**Receipts** tile.
+
+.. image:: media/purchase_triggering05.png
+    :align: center
+
+Select the receipt from your vendor and **Validate** it.
+
+.. image:: media/purchase_triggering06.png
+    :align: center
+
+Go back to the **Inventory** dashboard. In the delivery order, click on **#
+To Do**. The delivery order is now ready to ship.
+
+.. image:: media/purchase_triggering07.png
+    :align: center
+
+The status of the delivery changed from **Waiting Availability** to
+**Available**. Validate the transfer to confirm the delivery.
+
+.. image:: media/purchase_triggering08.png
+    :align: center
+
+.. seealso::
+    * :doc:`replenishment_methods`
+    * :doc:`setup_stock_rule`
+    * :doc:`warning_triggering`