[ADD] point_of_sale: How to register customers?

https://docs.google.com/document/d/1EsUviQZdwVV7zx6QQWzZVivPac9yWE6zoei2HiUciSw/edit#heading=h.teahchhagxca

point_of_sale/advanced/register.rst

@@ -1,3 +1,60 @@
 ==========================
 How to register customers?
-==========================
+==========================
+
+Register an order
+=================
+
+On the dashboard, click on **New Session**:
+
+.. image:: media/register01.png
+    :align: center
+
+You arrive now on the main view :
+
+.. image:: media/register02.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 quantity or weight by typing it on the keyboard.
+
+Record a customer
+=================
+
+On the main view, click on **Customer**:
+
+.. image:: media/register03.png
+    :align: center
+
+Register a new customer by clicking on the button.
+
+.. image:: media/register04.png
+    :align: center
+
+The following form appear. Fill in the relevant information:
+
+.. image:: media/register05.png
+    :align: center
+
+When it's done click on the **floppy disk** icon 
+
+.. image:: media/register06.png
+    :align: center
+
+You just registered a new customer. To set this customer to the current
+order, just tap on **Set Customer**.
+
+.. image:: media/register07.png
+    :align: center
+
+The customer is now set on the order.
+
+.. image:: media/register08.png
+    :align: center
+
+.. seealso::
+    * :doc:`../shop/cash_control`
+    * :doc:`../shop/invoice`
+    * :doc:`../shop/refund`
+    * :doc:`../shop/seasonal_discount`