[ADD] ecommerce: How to sell pricier product alternatives (upselling)

ecommerce/maximizing_revenue/upselling.rst

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+====================================================
+How to sell pricier product alternatives (upselling)
+====================================================
+
+In order to maximize your revenue, suggesting pricier alternative products is
+strongly advised for basic items. That way, your customer will spend more time
+browsing your catalog.
+
+.. image:: ./media/upsell_web.png
+   :align: center
+
+To do so:
+
+* Select such *Alternative Products* in the *Sales* tab of the product detail form.
+  3 alternatives are fine! Don't publish too many otherwise your customers will be confused.
+
+.. image:: ./media/upsell.png
+   :align: center
+
+* Turn on *Alternative Products* from the *Customize* menu of the product web page.
+
+.. image:: ./media/upsell_publish.png
+   :align: center
+
+