[ADD] accounting: How to manage withholding/retention taxes?

https://docs.google.com/document/d/1K4UP3ARZMRSM18WekcgvMRq2vUDDhvIHhKYgkL6XlqQ/edit

accounting/others/taxes/retention.rst

@@ -1,3 +1,70 @@
 ================================
 How to manage retention taxes?
 ================================
+
+A withholding tax, also called a retention tax, is a government
+requirement for the payer of a customer invoice to withhold or deduct
+tax from the payment, and pay that tax to the government. In most
+jurisdictions, withholding tax applies to employment income.
+
+With normal taxes, the tax is added to the subtotal to give you the
+total to pay. As opposed to normal taxes, withholding taxes are deducted
+from the amount to pay, as the tax will be paid by the customer.
+
+As, an example, in Colombia you may have the following invoice:
+
+.. image:: media/retention03.png
+   :align: center
+
+In this example, the **company** who sent the invoice owes $20 of taxes to
+the **government** and the **customer** owes $10 of taxes to the **government**.
+
+Configuration
+=============
+
+In Odoo, a withholding tax is defined by creating a negative tax. For a
+retention of 10%, you would configure the following tax (accessible
+through :menuselection:`Configuration --> Taxes`):
+
+.. image:: media/retention04.png
+   :align: center
+
+In order to make it appear as a retention on the invoice, you should set
+a specific tax group **Retention** on your tax, in the **Advanced Options**
+tab.
+
+.. image:: media/retention02.png
+   :align: center
+
+Once the tax is defined, you can use it in your products, sales order or
+invoices.
+
+.. tip::
+    If the retention is a percentage of a regular tax, create a Tax with a 
+    **Tax Computation** as a **Tax Group** and set the two taxes in this group 
+    (normal tax and retention).
+
+Applying retention taxes on invoices
+====================================
+
+Once your tax is created, you can use it on customer forms, sales order
+or customer invoices. You can apply several taxes on a single customer
+invoice line.
+
+.. image:: media/retention01.png
+   :align: center
+
+.. note::
+    When you see the customer invoice on the screen, you get only a 
+    **Taxes line** summarizing all the taxes (normal taxes & retentions). 
+    But when you print or send the invoice, Odoo does the correct 
+    grouping amongst all the taxes.
+
+The printed invoice will show the different amounts in each tax group.
+
+.. image:: media/retention03.png
+   :align: center
+
+.. todo::
+    seealso How to define new taxes?
+    seealso How to report on taxes due?