[ADD] point_of_sale: How to return and refund products?

https://docs.google.com/document/d/1LPYRm552oA2PWPZe9ex-Hr8sAfPnikmIWbu_b9oDsdA/edit#

point_of_sale/shop/refund.rst

@@ -1,3 +1,21 @@
 ==================================
 How to return and refund products?
 ==================================
+
+To refund a customer, from the PoS main view, you have to insert
+negative values. For instance in the last order you count too many
+``pumpkins`` and you have to pay back one.
+
+.. image:: media/refund01.png
+    :align: center
+
+You can see that the total is negative, to end the refund, you only have
+to process the payment.
+
+.. image:: media/refund02.png
+    :align: center
+
+.. seealso::
+    * :doc:`cash_control`
+    * :doc:`invoice`
+    * :doc:`seasonal_discount`