[ADD] point_of_sale: How to set up cash control?

https://docs.google.com/document/d/1eLMfQKXFB3xNIeHpfvwd26SUkRkTnz9JMbTI4f_req0/edit#

point_of_sale/shop/cash_control.rst

@@ -1,3 +1,98 @@
 ===========================
 How to set up cash control?
-===========================
+===========================
+
+Cash control permits you to check the amount of the cashbox at the
+opening and closing.
+
+Configuring cash control
+========================
+
+On the **Point of Sale** dashboard, 
+click on :menuselection:`More --> Settings`.
+
+.. image:: media/cash_control01.png
+    :align: center
+
+On this page, scroll and tick the the option **Cash Control**.
+
+.. image:: media/cash_control02.png
+    :align: center
+
+Starting a session
+==================
+
+On your point of sale dashboard click on **new session**:
+
+.. image:: media/cash_control03.png
+    :align: center
+
+Before launching the point of sale interface, you get the open control
+view. Click on set an opening balance to introduce the amount in the
+cashbox.
+
+.. image:: media/cash_control04.png
+    :align: center
+
+Here you can insert the value of the coin or bill, and the amount present in
+the cashbox. The system sums up the total, in this example we have
+``86,85€`` in the cashbox. Click on **confirm**.
+
+.. image:: media/cash_control05.png
+    :align: center
+
+You can see that the opening balance has changed and when you click on
+**Open Session** you will get the main point of sale interface.
+
+Register a sale
+===============
+
+.. image:: media/cash_control06.png
+    :align: center
+
+On the right you can see the list of your products with the categories
+on the top. If you click on a product, it will be added in the cart. You
+can directly set the correct quantity or weight by typing it on the
+keyboard.
+
+Payment
+=======
+
+Once the order is completed, click on **Payment**. You can choose the
+customer payment method. In this example, the customer owes you ``10.84€``
+and pays with a ``20€`` note. When it's done, click on **Validate**.
+
+.. image:: media/cash_control07.png
+    :align: center
+
+Your ticket is printed and you are now ready to make your second order.
+
+Closing a session
+=================
+
+At the time of closing the session, click on the **Close** button on the top
+right. Click again on the **Close** button to exit the point of sale interface. 
+On this page, you will see a summary of the transactions. At this moment you can
+take the money out.
+
+.. image:: media/cash_control08.png
+    :align: center
+
+For instance you want to take your daily transactions out of your
+cashbox.
+
+.. image:: media/cash_control09.png
+    :align: center
+
+Now you can see that the theoretical closing balance has been updated
+and it only remains you to count your cashbox to set a closing balance.
+
+.. image:: media/cash_control10.png
+    :align: center
+
+You can now validate the closing.
+
+.. seealso::
+    * :doc:`invoice`
+    * :doc:`refund`
+    * :doc:`seasonal_discount`