[ADD] point_of_sale: How to handle tips?

https://docs.google.com/document/d/151YD5CfsR7-imIJtNBKTnJt5qvk4HGYDkM2NQytrovU/edit#

point_of_sale/restaurant/tips.rst

@@ -1,3 +1,46 @@
 ===================
 How to handle tips?
-===================
+===================
+
+Configuration
+=============
+
+From the dashboard, click on :menuselection:`More --> Settings`.
+
+.. image:: media/tips01.png
+    :align: center
+
+Add a product for the tip.
+
+.. image:: media/tips02.png
+    :align: center
+
+In the tip product page, be sure to set a sale price of ``0€`` 
+and to remove all the taxes in the accounting tab.
+
+.. image:: media/tips03.png
+    :align: center
+
+Adding a tip
+============
+
+On the payment page, tap on **Tip**
+
+.. image:: media/tips04.png
+    :align: center
+
+Tap down the amount of the tip:
+
+.. image:: media/tips05.png
+    :align: center
+
+The total amount has been updated and you can now register the payment.
+
+.. image:: media/tips06.png
+    :align: center
+
+.. seealso::
+    * :doc:`../shop/cash_control`
+    * :doc:`../shop/invoice`
+    * :doc:`../shop/refund`
+    * :doc:`../shop/seasonal_discount`