[ADD] inventory: When should you use packages, packagings or units of measure ?

https://docs.google.com/document/d/1VIPgIwKaXtSoCOve7-rHCyi0_Hy-U22ypIRg58lRilY/edit?usp=sharing

inventory/settings/products.rst

@@ -9,3 +9,4 @@ Products
    products/uom
    products/variants
    products/usage
+   products/packages

inventory/settings/products/packages.rst

@@ -0,0 +1,3 @@
+====================
+How to use packages?
+====================

inventory/settings/products/usage.rst

@@ -1,3 +1,72 @@
 =======================================================
 When should you use packages, units of measure or kits?
 =======================================================
+
+Unit of measures
+================
+
+Units of measures are an indication about the unit used to handle a
+product. Products can be expressed in multiple units of measure at once.
+
+Activate this option if you are working with several ones in your
+warehouse.
+
+The purchase unit of measure might be different that the one you use in
+your warehouse.
+
+The selling unit of measure is set on the sale order and can be
+different.
+
+.. image:: media/usage01.png
+    :align: center
+
+.. note::
+    The conversion between the different units of measures is done
+    automatically. The only condition is that all the units have to be in
+    the same category (Unit, Weight, Volume, Length,...)
+
+Packages
+========
+
+The package is the physical container in which you put one or several
+product.
+
+.. image:: media/usage02.png
+    :align: center
+
+Packaging
+=========
+
+Packaging is the physical container that protects your product.
+
+If you are selling computers, the packaging contains the computer with
+the notice and the power plug.
+
+In Odoo, packagings are just used for indicative purpose.
+
+.. image:: media/usage03.png
+    :align: center
+
+.. note::
+    You can define on the **Packages** which **Packaging** it uses. But it is
+    only for indicative purpose.
+
+When to use packages, packagings or unit of measures ? 
+=======================================================
+
+For example, you are sellings eggs. In your warehouse, you manage the
+eggs individually. Lots of eggs are scrapped and you do it egg by egg.
+The **unit of measure** is ``Unit(s)``.
+
+If you are selling eggs by the dozen, the selling **unit of measure** is
+the ``Dozen``. You will set it on your sale order.
+
+The ``cardboard trays`` that contains the dozen of eggs is the **packaging**.
+
+When you are selling several trays, you might wrap all the trays into a
+``box`` or in a ``plastic`` wrapping. It is the **package**.
+
+.. seealso::
+    * :doc:`../../overview/start/setup`
+    * :doc:`uom`
+    * :doc:`packages`