[ADD] point_of_sale: How to apply manual discounts?

https://docs.google.com/document/d/1QIG_h7zvY6flk5qrjV6VLouDc3yR6o6cKlxrFtRO-uE/edit#heading=h.teahchhagxca

point_of_sale/advanced/manual_discount.rst

@@ -1,3 +1,86 @@
 ==============================
 How to apply manual discounts?
 ==============================
+
+Overview
+========
+
+You can apply manual discounts in two different ways. You can directly
+set a discount on the product or you can set a global discount on the
+whole cart.
+
+Discount on the product
+=======================
+
+On the dashboard, click on **New Session**:
+
+.. image:: media/manual_discount01.png
+    :align: center
+
+You will get into the main point of sale interface :
+
+.. image:: media/manual_discount02.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.
+
+The same way you insert a quantity, Click on **Disc** and then type the
+discount (in percent). This is how you insert a manual discount on a
+specific product.
+
+.. image:: media/manual_discount03.png
+    :align: center
+
+Global discount
+===============
+
+Configuration
+-------------
+
+If you want to set a global discount, you need to go to 
+:menuselection:`Configuration --> Settings` and 
+tick **Allow global discounts**
+
+.. image:: media/manual_discount04.png
+    :align: center
+
+Then from the dashboard, click on :menuselection:`More --> Settings`
+
+.. image:: media/manual_discount05.png
+    :align: center
+
+You have to activate **Order Discounts** and create a product that will be
+added as a product with a negative price to deduct the discount.
+
+.. image:: media/manual_discount06.png
+    :align: center
+
+On the product used to create the discount, set the price to ``0`` and do
+not forget to remove all the **taxes**, that can make the calculation wrong.
+
+.. image:: media/manual_discount07.png
+    :align: center
+
+Set a global discount
+---------------------
+
+Now when you come back to the **dashboard** and start a **new session**, a
+**Discount** button appears and by clicking on it you can set a **discount**.
+
+.. image:: media/manual_discount08.png
+    :align: center
+
+When it's validated, the discount line appears on the order and you can
+now process to the payment.
+
+.. image:: media/manual_discount09.png
+    :align: center
+
+.. seealso::
+    * :doc:`../shop/cash_control`
+    * :doc:`../shop/invoice`
+    * :doc:`../shop/refund`
+    * :doc:`../shop/seasonal_discount`