[ADD] accounting: How to define new taxes?

https://docs.google.com/document/d/1YgkZlbB5BxjmY-ft4bSZRGOm038L-8reFVnT_WMCnKs/edit#heading=h.aq4k0ooqiy2g

accounting/others/taxes.rst

@@ -6,4 +6,5 @@ Taxes
    :titlesonly:
 
    taxes/application
+   taxes/create
    taxes/retention

accounting/others/taxes/create.rst

@@ -0,0 +1,123 @@
+========================
+How to define new taxes?
+========================
+
+Odoo's tax engine is very flexible and support many different type of
+taxes: value added taxes (VAT), eco-taxes, federal taxes, retention,
+withholding taxes, etc. For most countries, the chart of account you
+setup is automatically pre-configured with the main taxes of the
+country.
+
+This section details how you can define new taxes for specific use
+cases.
+
+Managing Taxes
+==============
+
+Defining new taxes
+------------------
+
+In order to create new taxes, use the menu :menuselection:`Configuration --> Taxes` in the
+accounting application. From this menu, you will get all the taxes you
+can use: sales taxes and purchase taxes.
+
+.. image:: media/create01.png
+   :align: center
+
+When you create a new tax, you can set the following information:
+
+- **Scope**: Sales, Purchases or None (e.g. deprecated tax)
+
+- **Tax Account**: is the account where the tax journal item will be
+  posted. This field is optional, if you keep it empty, Odoo posts
+  the tax journal item in the income account.
+
+- **Label on Invoices**: a short text on how you want this tax to be
+  printed on invoice line. For example, a tax named "21% on
+  Services" can have the following label on invoice "21%".
+
+- **Tax Group**: defines where this tax is summed in the invoice footer.
+  All the tax belonging to the same tax group will be grouped on
+  the invoice footer. Examples of tax group: VAT, Retention.
+
+- **Include in Analytic Cost**: the tax is counted as a cost and, thus,
+  generate an analytic entry if your invoice uses analytic
+  accounts.
+
+- **Tags**: are used for custom reports. (usually, you can keep this field
+  empty)
+
+Taxes allow different computation mechanism:
+
+- **Fixed**, example: an eco-tax of $1.34 per battery
+
+- **Percentage**, example: 15% sales tax
+
+- **Percentage of price**, tax included: example in brazil
+
+- **Group of taxes**: allows to have a compound tax
+
+.. image:: media/create02.png
+   :align: center
+
+.. note::
+
+  if you need more advanced tax mechanism, you can install the
+  module **account_tax_python** and you will be able to define new taxes
+  with Python code.
+
+If a tax is included in price, it means that the price set on the
+product form includes this tax. As an example, let's say you have a
+product with a sale tax of 10%. The sale price on the product form is
+$100.
+
+- If the tax is not included in the price, you will get:
+
+  - Price without tax: $100
+
+  - Taxes: $10
+
+  - Total to pay: $110
+
+- If the tax is included in the price
+
+  - Price without tax: 90.91
+
+  - Taxes: $9.09
+
+  - Total to pay: $100
+
+.. image:: media/create03.png
+   :align: center
+
+Stop using a tax
+----------------
+
+If you want to avoid using a tax, you can not delete it because the tax
+is probably used in several invoices. So, in order to avoid users to
+continue using this tax, you should set the field **Tax Scope** to **None**.
+
+Setting your default taxes
+==========================
+
+Once you have created your taxes, you can define which tax is the
+default one when creating new products (for sales and purchase taxes).
+To set the default taxes, in the accounting application, go in the menu
+:menuselection:`Configuration --> Settings`.
+
+.. image:: media/create04.png
+   :align: center
+
+Once taxes are set by default, when you create a new product, Odoo will
+propose you the right taxes by default on the product form.
+
+.. note::
+
+  Setting the default taxes impact the creation of product, but not
+  sales order or invoices. The tax used on sales order or invoices is
+  defined by the product on the sale order line.
+
+.. seealso::
+
+  * :doc:`application`
+  * :doc:`retention`