[ADD] sales: How to invoice a support contract (prepaid hours)?

https://docs.google.com/document/d/1_zl7Hl3qh5_rIeA9jigfgQgpmGf8l0HLi2Rgl1p5H50/edit?usp=sharing

sales/invoicing/services/support.rst

@@ -1,3 +1,160 @@
 ==================================================
 How to invoice a support contract (prepaid hours)?
-==================================================
+==================================================
+
+There are different kinds of service sales: prepaid volume of hours/days
+(e.g. support contract), billing based on time and material (e.g.
+billing consulting hours) and a fixed price contract (e.g. a project).
+
+In this section, we will have a look at how to sell and keep track of a
+pre-paid support contract.
+
+As an example, you may sell a pack of ``50 Hours`` of support at ``$25,000``.
+The price is fixed and charged initially. But you want to keep track of
+the support service you did for the customer.
+
+Configuration
+=============
+
+Install the Sales and Timesheet applications
+--------------------------------------------
+
+In order to sell services, you need to install the **Sales** application,
+from the **Apps** icon. Install also the **Timesheets** application if you want
+to track support services you worked on every contract.
+
+.. image:: media/support01.png
+    :align: center
+
+.. image:: media/support02.png
+    :align: center
+
+Create Products
+---------------
+
+By default, products are sold by number of units. In order to sell
+services ``per hour``, you must allow using multiple unit of measures.
+From the **Sales** application, go to the menu 
+:menuselection:`Configuration --> Settings`.
+From this screen, activate the multiple **Unit of Measures** option.
+
+.. image:: media/support03.png
+    :align: center
+
+In order to sell a support contract, you must create a product for every
+support contract you sell. From the **Sales** application, use the menu
+:menuselection:`Sales --> Products`, create a new product with the following setup:
+
+-   **Name**: Technical Support
+
+-   **Product Type**: Service
+
+-   **Unit of Measure**: Hours
+
+-   **Invoicing Policy**: Ordered Quantities, since the service is
+    prepaid, we will invoice the service based on what has been
+    ordered, not based on delivered quantities.
+
+-   **Track Service**: Timesheet on contracts. An analytic account will
+    automatically be created for every order containing this service
+    so that you can track hours in the related account.
+
+.. image:: media/support04.png
+    :align: center
+
+.. tip:: 
+    There are different ways to track the service related to a sale 
+    order or product sold. With the above configuration, you can only 
+    sell one support contract per order. If your customer orders 
+    several service contracts on timesheet, you will have to split 
+    the quotation into several orders.
+
+Note that you can sell in different unit of measure than hours, example:
+days, pack of 40h, etc. To do that, just create a new unit of measure in
+the **Unit of Measure** category and set a conversion ratio compared to
+**Hours** (example: ``1 day = 8 hours``).
+
+Managing support contract
+=========================
+
+Quotations and Sale Orders
+--------------------------
+
+Once the product is created, you can create a quotation or a sale order
+with the related product. Once the quotation is confirmed and
+transformed into a sale order, your users will be able to record
+services related to this support contract using the timesheet
+application.
+
+.. image:: media/support05.png
+    :align: center
+
+Timesheets
+----------
+
+To track the service you do on a specific contract, you should use the
+timesheet application. An analytic account related to the sale order has
+been automatically created (``SO009 - Agrolait`` on the screenshot here
+above), so you can start tracking services as soon as it has been sold.
+
+.. image:: media/support06.png
+    :align: center
+
+Control delivered support on the sale order
+-------------------------------------------
+
+From the **Sales** application, use the menu 
+:menuselection:`Sales --> Sales Orders`  to control
+the progress of every order. On the sales order line related to the
+support contract, you should see the **Delivered Quantities** that are
+updated automatically, based on the number of hours in the timesheet.
+
+.. image:: media/support07.png
+    :align: center
+
+Upselling and renewal
+---------------------
+
+If the number of hours you performed on the support contract is bigger
+or equal to the number of hours the customer purchased, the sale order
+**Invoicing Status** is automatically set to **Upselling**. This means you
+have an opportunity to sell an extra contract to the customer since he
+used all his quota of service.
+
+Periodically (ideally once every two weeks), you should check the sales
+order that are in an upselling status. To do this, go to the 
+:menuselection:`Sales --> Sales Order` menu and use the **Upselling** 
+filter to get all orders that are in upselling.
+
+.. tip::
+    If you use Odoo CRM, a good practice is to create an opportunity for 
+    every sale order in upselling invoice status so that you easily track 
+    your upselling effort.
+
+If you sell an extra support contract, you can either add a new line on
+the existing sale order (thus, you continue to timesheet on the same
+order) or create a new order (thus, people will timesheet their hours on
+the new contract). To unmark the sale order as **Upselling**, you can set
+the sale order as done and it will disappear from your upselling list.
+
+Special Configuration
+=====================
+
+When creating the product form, you may set a different approach to
+track the service:
+
+-   **Create task and track hours**: in this mode, a task is created for
+    every sale order line. Then when you do the timesheet, you don't
+    record hours on a sale order/contract, but you record hours on a
+    task (that represents the contract). The advantage of this
+    solution is that it allows to sell several service contracts
+    within the same sale order.
+
+-   **Manually**: you can use this mode if you don't record timesheets in
+    Odoo. The number of hours you worked on a specific contract can
+    be recorded manually on the sale order line directly, in the
+    delivered quantity field.
+
+.. seealso::
+    * :doc:`time_materials`
+    * :doc:`../../../inventory/settings/products/uom`