[ADD] point_of_sale: How to accept credit card payments in Odoo POS using Mercury?

https://docs.google.com/document/d/1IudkCxdImPXfqpsnqb1fkxTCwp-abQy2vW8YPobf4CI/edit#

point_of_sale/advanced/mercury.rst

@@ -1,3 +1,97 @@
 =============================================================
 How to accept credit card payments in Odoo POS using Mercury?
-=============================================================
+=============================================================
+
+Overview
+========
+
+A **MercuryPay** account (see `MercuryPay website <https://www.mercurypay.com>`__.) 
+is required to accept credit
+card payments in Odoo 9 POS with an integrated card reader. MercuryPay
+only operates with US and Canadian banks making this procedure only
+suitable for North American businesses. An alternative to an integrated
+card reader is to work with a standalone card reader, copy the
+transaction total from the Odoo POS screen into the card reader, and
+record the transaction in Odoo POS.
+
+Module installation
+===================
+
+Go to **Apps** and install the **Mercury Payment Services** application.
+
+.. image:: media/mercury01.png
+    :align: center
+
+Mercury Configuration
+=====================
+
+In the **Point of Sale** application, click on 
+:menuselection:`Configuration --> Mercury Configurations` 
+and then on **Create**.
+
+.. image:: media/mercury02.png
+    :align: center
+
+Introduce your **credentials** and then save them.
+
+.. image:: media/mercury03.png
+    :align: center
+
+Then go to :menuselection:`Configuration --> Payment methods` 
+and click on **Create**. Under the **Point of Sale** tab you 
+can set a **Mercury configuration** to the **Payment method**.
+
+.. image:: media/mercury04.png
+    :align: center
+
+Finally, go to 
+:menuselection:`Configuration --> Point of Sale` and add 
+a new payment method on the point of sale by editing it.
+
+.. image:: media/mercury05.png
+    :align: center
+
+Then select the payment method corresponding to your mercury
+configuration.
+
+.. image:: media/mercury06.png
+    :align: center
+
+Save the modifications.
+
+Register a sale
+===============
+
+On the dashboard, you can see your point(s) of sales, click on 
+**New Session**:
+
+.. image:: media/mercury07.png
+    :align: center
+
+You will get the main point of sale interface:
+
+.. image:: media/mercury08.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 with credit cards
+=========================
+
+Once the order is completed, click on **Payment**. You can choose the credit
+card **Payment Method**.
+
+.. image:: media/mercury09.png
+    :align: center
+
+Type in the **Amount** to be paid with the credit card. Now you can swipe
+the card and validate the payment.
+
+.. seealso::
+    * :doc:`../shop/cash_control`
+    * :doc:`../shop/invoice`
+    * :doc:`../shop/refund`
+    * :doc:`../shop/seasonal_discount`