[ADD] sales: How to invoice milestones of a project?

https://docs.google.com/document/d/1NM7deV8LrpYSoLX58V-OLl25_OwoikS4BrR6EieeTco/edit#

sales/invoicing/services/milestones.rst

@@ -1,3 +1,127 @@
 =======================================
 How to invoice milestones of a project?
-=======================================
+=======================================
+
+There are different kind of service sales: prepaid volume of hours/days
+(e.g. support contract), billing based on time and material (e.g.
+billing consulting hours) or a fixed price contract (e.g. a project).
+
+In this section, we will have a look at how to invoice milestones of a
+project.
+
+Milestone invoicing can be used for expensive or large scale projects,
+with each milestone representing a clear sequence of work that will
+incrementally build up to the completion of the contract. For example, a
+marketing agency hired for a new product launch could break down a
+project into the following milestones, each of them considered as one
+service with a fixed price on the sale order :
+
+-  Milestone 1 : Marketing strategy audit - 5 000 euros
+
+-  Milestone 2 : Brand Identity - 10 000 euros
+
+-  Milestone 3 : Campaign launch & PR - 8 500 euros
+
+In this case, an invoice will be sent to the customer each time a
+milestone will be successfully reached. That invoicing method is
+comfortable both for the company which is ensured to get a steady cash
+flow throughout the project lifetime and for the client who can monitor
+the project's progress and pay in several times.
+
+.. note::
+    You can also use milestones to invoice percentages of the entire 
+    project. For example, for a million euros project, your company 
+    might require a 15% upfront payment, 30% at the midpoint and the 
+    balance at the contract conclusion. In that case, each payment will 
+    be considered as one milestone.
+
+Configuration
+=============
+
+Install the Sales application
+-----------------------------
+
+In order to sell services and to send invoices, you need to install the
+**Sales** application, from the **Apps** icon.
+
+.. image:: media/milestones01.png
+    :align: center
+
+Create products
+---------------
+
+In Odoo, each milestone of your project is considered as a product. From
+the **Sales** application, use the menu :menuselection:`Sales --> Products`, 
+create a new product with the following setup:
+
+-   **Name**: Strategy audit
+
+-   **Product Type**: Service
+
+-   **Invoicing Policy**: Delivered Quantities, since you will invoice
+    your milestone after it has been delivered
+
+-   **Track Service**: Manually set quantities on order, as you
+    complete each milestone, you will manually update their quantity
+    from the **Delivered** tab on your sale order
+
+.. image:: media/milestones02.png
+    :align: center
+
+.. note::
+    Apply the same configuration for the others milestones.
+
+Managing your project
+=====================
+
+Quotations and sale orders
+--------------------------
+
+Now that your milestones (or products) are created, you can create a
+quotation or a sale order with each line corresponding to one milestone.
+For each line, set the **Ordered Quantity** to ``1`` as each milestone is
+completed once. Once the quotation is confirmed and transformed into a
+sale order, you will be able to change the delivered quantities when the
+corresponding milestone has been achieved.
+
+.. image:: media/milestones03.png
+    :align: center
+
+Invoice milestones
+------------------
+
+Let's assume that your first milestone (the strategy audit) has been
+successfully delivered and you want to invoice it to your customer. On
+the sale order, click on **Edit** and set the **Delivered Quantity** of the
+related product to ``1``.
+
+.. tip::
+    As soon as the above modification has been saved, you will notice 
+    that the color of the line has changed to blue, meaning that the 
+    service can now be invoiced. In the same time, the invoice status 
+    of the SO has changed from **Nothing To Invoice** to **To Invoice**
+
+Click on **Create invoice** and, in the new window that pops up, select
+**Invoiceable lines** and validate. It will create a new invoice (in draft
+status) with only the **strategy audit** product as invoiceable.
+
+.. image:: media/milestones04.png
+    :align: center
+
+.. note::
+    In order to be able to invoice a product, you need to set up the 
+    **Accounting** application and to configure an accounting journal 
+    and a chart of account. Click on the following link to learn more:
+    :doc:`../../../accounting/overview/getting_started/setup`
+
+Back on your sale order, you will notice that the **Invoiced** column of
+your order line has been updated accordingly and that the **Invoice
+Status** is back to **Nothing to Invoice**.
+
+Follow the same workflow to invoice your remaining milestones.
+
+.. seealso::
+    * :doc:`recurring`
+    * :doc:`time_materials`
+    * :doc:`reinvoice`
+    * :doc:`support`