[ADD] ecommerce: How to create & share promotional codes

ecommerce/maximizing_revenue/promo_code.rst

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+=======================================
+How to create & share promotional codes
+=======================================
+
+Want to boost your sales for Xmas? Share promocodes through your marketing campaigns
+and apply any kind of discounts.
+
+Setup
+=====
+
+* Go to :menuselection:`Sales --> Settings` and choose
+  *Advanced pricing based on formula* for *Sale Price*.
+
+* Go to :menuselection:`Website Admin --> Catalog --> Pricelists` and create a new
+  pricelist with the discount rule (see :doc:`../managing_products/pricelists`).
+  Then enter a code.
+
+.. image:: ./media/promocode.png
+   :align: center
+
+* Make the promocode field available on your *Shopping Cart* page (option in
+  *Customize* menu). Add a product to cart to reach it.
+
+.. image:: ./media/promocode_publish.png
+   :align: center
+
+Once turned on you see a new section on the right side. On clicking *Apply* prices get
+automatically updated in the cart.
+
+.. image:: ./media/promocode_web.png
+   :align: center
+
+The promocode used by the customer is stored in the system so you can analyze the
+performance of your marketing campaigns.
+
+.. image:: ./media/promocode_backend.png
+   :align: center
+
++ Show sales per pricelists...
+
+.. seealso::
+
+  * :doc:`../managing_products/pricelists`