[ADD] point_of_sale: How to use discount tags on products?

https://docs.google.com/document/d/1RqPb-aY0X4Fdlk6KktYG6w13SCtdk9SKJQkiYA5LEZ0/edit#

point_of_sale/advanced/discount_tags.rst

@@ -1,3 +1,64 @@
 =====================================
 How to use discount tags on products?
-=====================================
+=====================================
+
+This tutorial will describe how to use discount tags on products.
+
+Barcode Nomenclature
+====================
+
+To start using discounts tags, let's first have a look at the **barcode
+nomenclature** in order to print our correct discounts tags.
+
+I want to have a discount for the product with the following barcode.
+
+.. image:: media/discount_tags01.png
+    :align: center
+
+Go to :menuselection:`Point of Sale --> Configuration --> Barcode Nomenclatures`. 
+In the default nomenclature, you can see that to set a discount, you have to
+start you barcode with ``22`` and the add the percentage you want to set for
+the product. 
+
+.. image:: media/discount_tags02.png
+    :align: center
+
+For instance if you want ``50%`` discount on a product you have to start you
+barcode with ``2250`` and then add the product barcode. In our example, the
+barcode will be:
+
+.. image:: media/discount_tags03.png
+    :align: center
+
+Scanning your products
+======================
+
+If you go back to the **dashboard** and start a **new session**
+
+.. image:: media/discount_tags04.png
+    :align: center
+
+You have to scan:
+
+1. the product
+
+2. the discount tag
+
+When the product is scanned, it appears on the ticket
+
+.. image:: media/discount_tags05.png
+    :align: center
+
+Then when you scan the discount tag, ``50%`` discount is applied on the
+product.
+
+.. image:: media/discount_tags06.png
+    :align: center
+
+That's it, this how you can use discount tag on products with Odoo.
+
+.. seealso::
+    * :doc:`../shop/cash_control`
+    * :doc:`../shop/invoice`
+    * :doc:`../shop/refund`
+    * :doc:`../shop/seasonal_discount`