[ADD] point_of_sale: How to apply Time-limited or seasonal discounts?

https://docs.google.com/document/d/1OMJDCvxhy0wGBtUjJHg81uIojdcq5yZmsKTsb322u9k/edit#

point_of_sale/shop/seasonal_discount.rst

@@ -1,3 +1,93 @@
 ================================================
 How to apply Time-limited or seasonal discounts?
-================================================
+================================================
+
+Configuration
+=============
+
+To apply time-limited or seasonal discount, use the pricelists.
+
+You have to create it and to apply it on the point of sale.
+
+Sales application configuration
+-------------------------------
+
+In the **Sales** application, go to
+:menuselection:`Configuration --> Settings`. Tick
+**Advanced pricing based on formula**.
+
+.. image:: media/seasonal_discount01.png
+    :align: center
+
+Creating a pricelist
+--------------------
+
+Once the setting has been applied, a **Pricelists** section appears under 
+the configuration menu on the sales application.
+
+.. image:: media/seasonal_discount02.png
+    :align: center
+
+Click on it, and then on **Create**.
+
+.. image:: media/seasonal_discount03.png
+    :align: center
+
+Create a **Pricelist** for your point of sale. Each pricelist can contain
+several items with different prices and different dates. It can be done
+on all products or only on specific ones. Click on **Add an item**.
+
+.. image:: media/seasonal_discount04.png
+    :align: center
+
+.. demo:fields:: product.product_pricelist_action2
+
+For example, the price of the oranges costs ``3€`` but for two days, we want
+to give a ``10%`` discount to our PoS customers.
+
+.. image:: media/seasonal_discount05.png
+    :align: center
+
+You can do it by adding the product or its category and applying a
+percentage discount. Other price computation can be done for the
+pricelist.
+
+After you save and close, your pricelist is ready to be used.
+
+.. image:: media/seasonal_discount06.png
+    :align: center
+
+Applying your pricelist to the Point of Sale
+--------------------------------------------
+
+From the dashboard, click on :menuselection:`More --> Settings`.
+
+.. image:: media/seasonal_discount07.png
+    :align: center
+
+On the right, you will be able to assign a pricelist.
+
+.. image:: media/seasonal_discount08.png
+    :align: center
+
+.. note:: 
+    You just have to update the pricelist to apply the time-limited
+    discount(s).
+
+Register an order
+=================
+
+When you start a new session, you can see that the price have
+automatically been updated.
+
+.. image:: media/seasonal_discount09.png
+    :align: center
+
+.. note::
+    When you update a pricelist, you have to close and open the
+    session.
+
+.. seealso::
+    * :doc:`cash_control`
+    * :doc:`invoice`
+    * :doc:`refund`