[ADD] accounting: Add a 'Set up cash roundings' page

Original document: https://docs.google.com/document/d/13Td5QE_2SszfpqavquQJX4JlcEThmCgRA_bIs3AusKY/edit#heading=h.4w195d55wcf9

accounting/receivables/customer_invoices.rst

@@ -7,6 +7,7 @@ Customer Invoices
 
    customer_invoices/overview
    customer_invoices/cash_discounts
+   customer_invoices/cash_rounding
    customer_invoices/payment_terms
    customer_invoices/installment_plans
    customer_invoices/deferred_revenues

accounting/receivables/customer_invoices/cash_rounding.rst

@@ -0,0 +1,52 @@
+Set up cash roundings
+=====================
+
+In some currencies, the smallest coins do not exist. For example, in
+Switzerland, there is no coin for 0.01 CHF. For this reason, if invoices
+are paid in cash, you have to round their total amount to the smallest
+coin that exist in the currency. For the CHF, the smallest coin is 0.05
+CHF.
+
+There are two strategies for the rounding:
+
+1. Add a line on the invoice for the rounding
+
+2. Add the rounding in the tax amount
+
+Both strategies are applicable in Odoo.
+
+Configuration 
+--------------
+
+First, you have to activate the feature. For this, go in
+:menuselection:`Accounting --> Configuration --> Settings`
+and activate the Cash Rounding.
+
+.. image:: media/cash_rounding01.png
+    :align: center
+
+There is a new menu to manage cash roundings in
+:menuselection:`Accounting --> Configuration --> Management --> Cash roundings`.
+
+Now, you can create cash roundings. You can choose between two rounding
+strategies:
+
+1. **Add a rounding line**: if a rounding is necessary, Odoo will add a
+   line on your customer invoice to take this rounding into
+   account. You also have to define the account in which the
+   rounding will go.
+
+2. **Modify tax amount:** Odoo will add the rounding to the amount of
+   the highest tax.
+
+.. image:: media/cash_rounding02.png
+    :align: center
+
+Apply roundings
+---------------
+
+Once your roundings are created, you can apply them on customer
+invoices. On the customer invoices, there is a new field called **Cash
+Rounding Method** where you can simply choose one of the rounding methods
+created previously. If needed, a rounding will be applied to the
+invoice.