[ADD] point_of_sale: How to invoice from the POS interface?

https://docs.google.com/document/d/1Rr1tmje_y8GSFIVYHZW43lsnX86f_ZTpcb2peVaLw9Q/edit

point_of_sale/shop/invoice.rst

@@ -1,3 +1,84 @@
 ======================================
 How to invoice from the POS interface?
-======================================
+======================================
+
+Register an order
+=================
+
+On the **Dashboard**, you can see your points of sales, click on **New
+session**:
+
+.. image:: media/invoice01.png
+    :align: center
+
+You are on the ``main`` point of sales view :
+
+.. image:: media/invoice02.png
+    :align: center
+
+On the right you can see the list of your products with the categories
+on the top. Switch categories by clicking on it.
+
+If you click on a product, it will be added in your cart. You can
+directly set the correct **Quantity/Weight** by typing it on the keyboard.
+
+Add a customer
+==============
+
+On the main view, click on **Customer** (above **Payment**):
+
+.. image:: media/invoice03.png
+    :align: center
+
+You must set a customer in order to be able to issue an invoice.
+
+.. image:: media/invoice04.png
+    :align: center
+
+You can search in the list of your customers or create new ones by
+clicking on the icon.
+
+.. image:: media/invoice05.png
+    :align: center
+
+.. note::
+    For more explanation about adding a new customer. Please read the
+    document :doc:`../advanced/register`.
+
+Payment and invoicing
+=====================
+
+Once the cart is processed, click on **Payment**. You can choose the
+customer payment method. In this example, the customer owes you ``10.84 €``
+and pays with by a ``VISA``.
+
+Before clicking on **Validate**, you have to click on **Invoice** in order to
+create an invoice from this order.
+
+.. image:: media/invoice06.png
+    :align: center
+
+Your invoice is printed and you can continue to make orders.
+
+Retrieve invoices of a specific customer
+========================================
+
+To retrieve the customer’s invoices, go to the **Sale** application, click
+on :menuselection:`Sales --> Customers`.
+
+On the customer information view, click on the **Invoiced** button :
+
+.. image:: media/invoice07.png
+    :align: center
+
+You will get the list all his invoices. Click on the invoice to get the
+details.
+
+.. image:: media/invoice08.png
+    :align: center
+
+.. seealso::
+    * :doc:`cash_control`
+    * :doc:`../advanced/register`
+    * :doc:`refund`
+    * :doc:`seasonal_discount`