[ADD] accounting: How to define an installment plan for an invoice

https://docs.google.com/document/d/1nnnCROHu2HU_l0fsvQlfmq3If0W8TPg9Vy4-3Jg4TJI/edit?pli=1#heading=h.rnc4w7jho1pm

accounting/receivables/customer_invoices/installment_plans.rst

@@ -1,3 +1,91 @@
 ================================================
 How to define an installment plan for an invoice
 ================================================
+In order to manage installment plans related to an invoice, you should
+use payment terms in Odoo. They apply on both customer invoices and
+supplier bills.
+
+Example, for a specific invoice:
+
+-  Pay 50% within 10 days
+-  Pay the remaining balance within 30 days
+
+.. note::
+
+	payment terms are not to be confused with a payment in several parts. If,
+	for a specific order, you invoice the customer in two parts, that’s not a
+	payment term but an invoice policy.
+
+Configuration
+=============
+
+Configure your usual installment plans from the application :menuselection:`Accounting -->
+Configuration > Payment Terms`.
+
+A payment term may have one line (ex: 21 days) or several lines (10%
+within 3 days and the balance within 21 days). If you create a payment
+term with several lines, make sure the latest one is the balance. (avoid
+doing 50% in 10 days and 50% in 21 days because, with the rounding, it
+may not compute exactly 100%)
+
+.. image:: ./media/installment01.png
+
+.. tip::
+
+	The description of the payment term will appear on the invoice or the sale order.
+
+Payment terms for customers
+===========================
+
+You can set payment terms on:
+
+- **a customer**: the payment term automatically applies on new sales
+  orders or invoices for this customer. Set payment terms on
+  customers if you grant this payment term for all future orders
+  for this customer.
+
+- **a quotation**: the payment term will apply on all invoices created
+  from this quotation or sale order, but not on other quotations
+
+- **an invoice**: the payment term will apply on this invoice only
+
+If an invoice contains a payment term, the journal entry related to the
+invoice is different. Without payment term, an invoice of $100 will
+produce the following journal entry (for the clarity of the example, we
+did not set any tax on the invoice):
+
++----------------------+------------+---------+----------+
+| Account              | Due date   | Debit   | Credit   |
++======================+============+=========+==========+
+| Account Receivable   |            | 100     |          |
++----------------------+------------+---------+----------+
+| Income               |            |         | 100      |
++----------------------+------------+---------+----------+
+
+If you do an invoice the 1st of January with a payment term of 10%
+within 3 days and the balance within 30 days, you get the following
+journal entry:
+
++----------------------+------------+---------+----------+
+| Account              | Due date   | Debit   | Credit   |
++======================+============+=========+==========+
+| Account Receivable   | Jan 03     | 10      |          |
++----------------------+------------+---------+----------+
+| Account Receivable   | Jan 30     | 90      |          |
++----------------------+------------+---------+----------+
+| Income               |            |         | 100      |
++----------------------+------------+---------+----------+
+
+On the customer statement, you will see two lines with different due
+dates. To get the customer statement, use the menu Sales > Customers
+Statement.
+
+.. seealso::
+
+	* :doc:`overview`
+	* :doc:`payment_terms`
+
+.. rst-class:: text-muted
+
+| Written by Fabien Pinckaers (Odoo)
+| Proofread by Jordi Ballester (eficent)