[ADD] account: How to setup cash discounts?

https://docs.google.com/document/d/1d3RRYzYyUbgQFDPvY9M8GS1_vGWxMRBGDszPdVB3Hq0/edit#heading=h.456xgexhcza7

accounting/receivables/customer_invoices/cash_discounts.rst

@@ -1,3 +1,131 @@
 ============================
 How to setup cash discounts?
 ============================
+
+Cash discounts are an incentive (usually a small percentage) that you
+offer to customers in return for paying a bill owed before the scheduled
+due date. If used properly, cash discounts improve the Days Sales
+Outstanding aspect of a business's cash conversion cycle.
+
+For example, a typical cash discount would be: you offer a 2% discount
+on an invoice due in 30 days if the customer were to pay within the
+first 5 days of receiving the invoice.
+
+Configuration
+=============
+
+Payment terms
+-------------
+
+In order to manage cash discounts, we will use the payment terms
+concept of Odoo (From the Accounting module, go to :menuselection`Configuration -->
+Management --> Payment terms --> Create`).
+
+Let's start with the above example: a 2% discount on an invoice due in
+30 days if the customer were to pay within the first 5 days.
+
+A typical payment term of 30 days would have only one installment:
+balance in 30 days. But, in order to configure the cash discount, you
+can configure the payment term with two installments:
+
+-  98% within 5 days
+-  balance within 30 days
+
+.. image:: ./media/discount01.png
+   :align: center
+
+To make it clear that it's not a payment term but a cash discount, don't
+forget to set a clear description that will appear on the invoice:
+Invoice is due within 30 days, but you can benefit from a 2% cash
+discount if you pay within 5 days.
+
+Bank reconciliation model
+-------------------------
+
+In order to speed up the bank reconciliation process, we can create a
+model of entry for all cash discounts. To do that, from the Accounting
+application dashboard, click on the "More" link on the bank and choose
+the option "Reconciliation Models".
+
+.. image:: ./media/discount02.png
+   :align: center
+
+Create a new model for cash discounts as follow:
+
+-  **Button Label**: Cash Discount
+-  **Account**: Cash Discount (according to your country)
+-  **Amount Type**: Percentage
+-  **Amount**: 100%
+-  **Taxes**: depending on your country, you may put a tax on the cash
+       discount if taxes have to be deduced
+
+.. image:: ./media/discount03.png
+   :align: center
+
+.. tip::
+	
+	Even if it's a 2% cash discount, set a 100% amount on the reconciliation model
+	as it means 100% of the remaining balance (the 2%). You can use the same
+	reconciliation model for all your cash discount. No need to create a model
+	per payment term.
+
+Creating an invoice with a cash discount
+========================================
+
+When you create a customer invoice, set the right payment term "30 days,
+2% cash discount" right after having selected the customer.
+
+.. image:: ./media/discount04.png
+   :align: center
+
+Once the invoice is validated, Odoo will automatically split the account
+receivable part of the journal entry with two installments having a
+different due date: 98% within 5 days, 2% within 30 days.
+
+.. image:: ./media/discount05.png
+   :align: center
+
+Payment
+=======
+
+Paying the invoice with a cash discount
+---------------------------------------
+
+If the customer pays with a cash discount, when processing the bank
+statement, you will match the payment (98%) with the related line in the
+journal entry.
+
+.. image:: ./media/discount06.png
+   :align: center
+
+As you can see in the above screenshot, when selecting the customer, you
+also see the 2% remaining of 3$. If you want to accept the cash discount
+(if the customer paid within the 5 days), you can click on this line
+with 2%, click on "Open Balance", and select your "Cash Discount"
+reconciliation model. That way, the invoice is marked as fully paid.
+
+.. note::
+
+	from now on, matching the remaining 2% has to be done manually. In the future,
+	we plan to automate the reconciliation of the 2% if the 98% are paid on time.
+
+Paying the invoice in full
+--------------------------
+
+If the customer pays the invoice fully, without benefiting from the cash
+discount, you will reconcile the payment (in full) with the two lines
+from the invoice (98% and 2%). Just click on the two lines to match them
+with the payment.
+
+.. image:: ./media/discount07.png
+   :align: center
+
+Related topics
+==============
+
+-  Overview of the invoicing processes
+
+.. rst-class:: text-muted
+
+| Written by Fabien Pinckaers (Odoo)
+| Proofread by Geoffrey Bressan (Odoo)