[ADD] ecommerce: How to sell accessories and optional products (cross-selling)

ecommerce/maximizing_revenue.rst

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+=================
+Maximize your revenue
+=================
+
+.. toctree::
+   :titlesonly:
+
+   maximizing_revenue/upselling
+   maximizing_revenue/cross_selling
+   maximizing_revenue/reviews
+   maximizing_revenue/pricing
+   maximizing_revenue/promo_code
+
+
+
+

ecommerce/maximizing_revenue/cross_selling.rst

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+=============================================================
+How to sell accessories and optional products (cross-selling)
+=============================================================
+
+You sell computers. Why not stimulating your customers to buy a top-notch
+screen or an extra-warranty? That's the goal of cross-selling functionalities: 
+
+* Accessory products on checkout page,
+* Optional products on a new *Add to Cart* screen (not installed by default).
+
+Accessory products when checking out
+===================================
+
+Accessories (e.g. for computers: mouse, keyboard) show up when the customer
+reviews the cart before paying.
+
+.. image:: ./media/cross_sell_accessory_web.png
+   :align: center
+
+Select accessories in the *Sales* tab of the product detail page.
+
+.. image:: ./media/cross_sell_accessory.png
+   :align: center
+
+.. note::
+    There is an algorithm to figure out the best accessories to display in case
+    several items are added to cart. If any item is the accessory of several products
+    added to cart, it is most likely that it will be atop the list of suggested accessories.
+
+Optional products when adding to cart
+=====================================
+
+Optional products are directly related to the item added to cart (e.g. for computers:
+warranty, OS software, extra components). Whenever the main product is added to cart,
+such a new screen pops up as an extra step.
+
+.. image:: ./media/cross_sell_optional_web.png
+   :align: center
+
+To publish optional products:
+
+* Install *eCommerce Optional Products* addon in *Apps* menu. Remove the default filter to
+search on addons as well, otherwise only main apps show up.
+
+.. image:: ./media/cross_sell_optional_install.png
+   :align: center
+
+* Select optional items from the *Sales* tab of the product detail form.
+
+.. image:: ./media/cross_sell_optional.png
+   :align: center
+
+.. note::
+    The quantity of optional items added to cart is the same than the main item.
+