[ADD] inventory: How to send products to customers directly from suppliers (drop-shipping)?

https://docs.google.com/document/d/106ndy_rKphwL8dsXKPOT71TZeM7WGjdY_YlynCgsqX0/edit

inventory/delivery.rst

@@ -10,4 +10,5 @@ Delivery Orders
    delivery/one_step
    delivery/two_steps
    delivery/three_steps
-   delivery/scheduled_dates
+   delivery/scheduled_dates
+   delivery/dropshipping

inventory/delivery/dropshipping.rst

@@ -0,0 +1,107 @@
+==========================================================================
+How to send products to customers directly from suppliers (drop-shipping)?
+==========================================================================
+
+What is drop-shipping?
+----------------------
+
+Drop-Shipping is a system that allows orders taken from your store to be
+shipped straight from your supplier to your customer. On a usual
+delivery system, products are sent from your supplier to your warehouse
+to be put in stock, and then shipped to your customers after ordering.
+With drop-shipping, no item is stocked. When a customer places an order
+in your shop, the item is delivered straight from the supplier to the
+customer. Therefore, the product doesn’t need to get through your
+warehouse.
+
+Points to be considered while implementing drop-shipping
+--------------------------------------------------------
+
+-  Use drop-shipping only for the products you can't or don't want to
+   keep in stock. One reason is that you'll always make smaller
+   margins on items that are drop-shipped, so you should keep it
+   only for items that take up a lot of space in your warehouse.
+
+-  Drop-shipping is best for niche products. Chances are that products
+   that are in high demand are being offered by large suppliers at a
+   fraction of the price you'll be able to charge, so using a more
+   costly shipping method won't be financially rewarding. But if
+   your product is unique, then it makes sense!
+
+-  To protect your customers from bad experiences, test drop-shipping
+   companies for yourself beforehand and list the best ones.
+
+-  Make sure time is not against you. Drop-shipping should take a
+   reasonable amount of time and surely not more than it would have
+   taken you to handle it all by yourself. It's also nice to be able
+   to provide your customers with a tracking number.
+
+-  Items have to be available from your supplier. It's good to know if
+   the product you're selling is available upstream. If you don't
+   have that information, inform your customers that you don't hold
+   the item in stock and that it's subject to availability from a
+   third party.
+
+.. tip::
+    For more information and insights on Drop-shipping, read our blog on 
+    `What is drop-shipping and how to use it <https://www.odoo.com/blog/business-hacks-1/post/what-is-drop-shipping-and-how-to-use-it-250>`__.
+
+Configuring drop-shipping
+-------------------------
+
+-   Open the menu :menuselection:`Inventory --> Configuration --> Settings`. 
+    Go to **Location & Warehouse**, locate the **Dropshipping**
+    option and tick the box **Allow suppliers to deliver directly to
+    your customers**. Then, click on *Apply**.
+
+.. image:: media/dropshipping06.png
+   :align: center
+
+-  Then go to the menu :menuselection:`Sales --> Configuration --> Settings`.
+   Locate **Order Routing** and tick the box **Choose specific
+   routes on sales order lines (advanced)**. Click on **Apply**.
+
+.. image:: media/dropshipping05.png
+   :align: center
+
+-  Now, open the menu :menuselection:`Sales --> Sales --> Products`. 
+   Add a supplier to the products you want to dropship.
+
+.. image:: media/dropshipping03.png
+   :align: center
+
+How to send products from the customers directly to the suppliers
+=================================================================
+
+-  Create a **Sales Order** and specify on a sales order line for your
+   products that the route is **Dropshipping**.
+
+.. image:: media/dropshipping01.png
+   :align: center
+
+-  Open the menu :menuselection:`Purchases --> Purchase --> Requests for Quotation`.
+   The draft purchase order is automatically created from the
+   procurement with the drop-shipping route. Once the order is
+   confirmed, you will see that one shipment has been created.
+
+.. image:: media/dropshipping04.png
+   :align: center
+
+-  To confirm the sending from the vendor to the customer, go back to
+   **Inventory** app. On the dashboard, click on **# TO RECEIVE** on
+   the dropship card.
+
+.. image:: media/dropshipping02.png
+   :align: center
+
+-  It will open the list of drop-shipping transfers. Validate the
+   transfer once it has been done. The items will be directly
+   delivered from the partner to the customer without transiting to
+   your warehouse.
+
+.. seealso::
+    * :doc:`inventory_flow`
+
+.. todo::
+    Add link to this section when available
+    * How to analyse the performance of my vendors?