[ADD] purchase: How to setup drop-shipping?

https://docs.google.com/document/d/1k7j6aa1iq4h03WTMelwHs3_ol4npO_nVJUCAB07J9X0/edit?usp=sharing

purchase/replenishment/flows/dropshipping.rst

@@ -2,4 +2,45 @@
 How to setup drop-shipping?
 ===========================
 
-*Coming soon...*
+Overview
+========
+
+Drop shipping allows to deliver the goods directly from the supplier to
+the customer. It means that the products does not transit through your
+stock.
+
+Configuration
+=============
+
+First, configure the **Routes** and **Dropshipping**. Go to 
+:menuselection:`Inventory --> Configuration --> Settings`. 
+Check **Advanced routing of products using rules** in the 
+**Routes** section and **Allow suppliers to deliver directly to
+your customers** in the **Drop Shipping** section.
+
+.. image:: media/dropshipping01.png
+    :align: center
+
+You have to allow the choice of the route on the sale order. Go to the
+**Sales** application, :menuselection:`Configuration --> Settings` and 
+tick **Choose specific routes on sales order lines (advanced)**.
+
+.. image:: media/dropshipping02.png
+    :align: center
+
+How to use drop-shipping? 
+=========================
+
+Create the sale order and select the route as **Dropshipping** on the
+concerned order lines.
+
+.. image:: media/dropshipping03.png
+    :align: center
+
+Once the order has been confirmed, no move will be created from your
+stock. The goods will be delivered directly from your vendor to your
+customer.
+
+.. note::
+    In order to be able to invoice the delivery, you must set the
+    invoice policy of your product on **Ordered quantities**.