[ADD] purchase: Dropship Products

Origin document: https://docs.google.com/document/d/1ejDrzr4dKYIjBD6M6OsIwEvcMsBHmhN_F83toM3nAOE/edit

purchase/purchases/rfq.rst

@@ -14,3 +14,4 @@ Request for Quotation and Purchase Orders
    rfq/analyze
    rfq/3_way_matching
    rfq/warning
+   rfq/dropshipping

purchase/purchases/rfq/dropshipping.rst

@@ -0,0 +1,54 @@
+=================
+Dropship products
+=================
+
+Dropshipping is a method in which the vendor does not keep products in
+stock but instead transfers the products directly from the supplier to
+the customer.
+
+.. note::
+   You need the *Sales*, *Inventory* and *Purchases* modules
+   for this feature to work.
+
+Activate the dropshipping feature
+=================================
+
+Go to :menuselection:`Purchases --> Configuration --> Settings` and
+select *Dropshipping*.
+
+.. image:: media/dropshipping01.png
+   :align: center
+
+A new route *Dropshipping* has been created. You can go on a product
+and tick the dropshipping route. You also need to define to which vendor
+you will buy the product. Now, each time this product will be sold, it
+will be drop shipped.
+
+.. image:: media/dropshipping02.png
+   :align: center
+
+If you don't always dropship that product, you can go to
+:menuselection:`Sales --> Configuration --> Settings` and tick the box
+in front of *Order-Specific Routes*.
+
+.. image:: media/dropshipping03.png
+   :align: center
+
+When you create a quotation or sales order, you can now decide, order
+line per order line, which products should be drop shipped by selecting
+the dropship route on the SO line.
+
+.. image:: media/dropshipping04.png
+   :align: center
+
+Drop ship a product
+===================
+
+Once a sale order with products to dropship is confirmed, a request for
+quotation is generated by Odoo to buy the product to the vendor. If you
+confirm that request for quotation into a purchase order, it will create
+a transfer from your vendor directly to your customer. The products
+doesn't go through your own stock.
+
+.. image:: media/dropshipping05.png
+   :align: center