[ADD] accounting: add new doc about the "Verify VAT numbers" feature (#692)

accounting/fiscality/taxes.rst

@@ -13,3 +13,4 @@ Taxes
    taxes/retention
    taxes/B2B_B2C
    taxes/cash_basis_taxes
+   taxes/vat_validation

accounting/fiscality/taxes/vat_validation.rst

@@ -2,3 +2,42 @@
 VIES VAT numbers validation
 ===========================
 
+**VAT Information Exchange System** - abbreviated **VIES** - is a tool provided by the European
+Commission that allows you to check the validity of VAT numbers of companies registered in the
+European Union.
+
+Odoo provides a feature to **Verify VAT Numbers** when you save a contact. This helps you make sure
+that your contacts provided you with a valid VAT number without leaving Odoo interface.
+
+Configuration
+=============
+
+To enable this feature, go to :menuselection:`Accounting --> Configuration --> Settings --> Taxes`,
+enable the **Verify VAT Numbers** feature, and click on *Save*.
+
+.. image:: media/vat-validation-configuration.png
+   :align: center
+   :alt: Enable "Verify VAT Numbers" in Odoo Accounting
+
+VAT Number validation
+=====================
+
+Whenever you create or modify a contact, make sure to fill out the **Country** and **VAT** fields.
+
+.. image:: media/vat-validation-contact-form.png
+   :align: center
+   :alt: Fill out the contact form with the country and VAT number before clicking on *Save*
+
+When you click on *Save*, Odoo runs a VIES VAT number check, and displays an error message if the
+VAT number is invalid.
+
+.. image:: media/vat-validation-error.png
+   :align: center
+   :alt: Odoo displays an error message instead of saving when the VAT number is invalid
+
+.. important::
+   This tool checks the VAT number's validity but does not check the other fields' validity.
+
+.. seealso::
+
+   - `European Commission: VIES search engine <https://ec.europa.eu/taxation_customs/vies/vatRequest.html>`__