[ADD] purchase: How to purchase in different units of measures than sales?

https://docs.google.com/document/d/1afmogMdbhAHXbnTWcTgaT8aUK2cK90bI7rMxlTILWmM/edit#heading=h.g3uswfy4vhyu

purchase/purchases/master/uom.rst

@@ -1,3 +1,136 @@
 =========================================================
 How to purchase in different unit of measures than sales?
-=========================================================
+=========================================================
+
+In day-to-day business, it may happen that your supplier uses a
+different unit of measure than you do in sales. This can cause confusion
+between sales and purchase representative and even make you lose a lot
+of time converting measures. Luckily in Odoo, you can handle different
+units of measures between sales and purchase very easily.
+
+Let's take the following examples:
+
+1)  You buy water from a supplier. The supplier is american and sells his
+    water in **Gallons**. Your customers however are European. You would
+    thus like to see your purchases quantities expressed in **Gallons**
+    and the sold quantities in **Liters**.
+
+2)  You buy curtains from a supplier. The supplier sells you the curtains
+    in the unit **roll** and you sell the curtains in **square meters**.
+
+Configuration
+=============
+
+Install purchase and sales modules
+----------------------------------
+
+The first step is to make sure that the apps **Sales** and **Purchase** are
+correctly installed.
+
++------------+------------+
+|  |uom01|   |  |uom02|   |
++============+============+
++------------+------------+
+
+Enable the Unit of Measures option
+----------------------------------
+
+Enter the purchase module, select :menuselection:`Configuration --> Settings` 
+and tick the **Some products may be sold/purchased in different unit of measures
+(advanced)** box.
+
+.. image:: media/uom08.png
+    :align: center
+
+Specify sales and purchase unit of measures
+-------------------------------------------
+
+Standard units of measures
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Let's take the classic units of measures existing in Odoo as first
+example. Please remember that differents units of measures between sales
+and purchase necessarily need to share the same category. Categories
+include: **Unit**, **weight**, **working time**, **volume**, etc.
+
+.. tip::
+    It is possible to create your own category and unit of measure if 
+    it is not standard in Odoo (see next chapter).
+
+Let's assume we buy water from our vendors in **Gallons** and sell to our
+customers in **Liters**.
+
+We go into the purchase module select :menuselection:`Purchase --> Products`.
+
+Create your own product or select an existing one. In the products
+general information you have the possibility to select the **Unit of
+measure** (will be used in sales, inventory,...) and the **Purchase Unit
+of Measure** (for purchase).
+
+In this case select **Liters** for **Unit of Measure** and **Gallons** for
+**Purchase Unit of Measure**.
+
+.. image:: media/uom05.png
+    :align: center
+
+Create your own unit of measure and unit of measure category
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Let's take now our second example (you buy curtains from a supplier, the
+supplier sells you the curtains in the unit **roll** and you sell the
+curtains in **square meters**).
+
+The two measures are part of two different categories. Remember, you
+cannot relate an existing measure from one category with an existing
+measure of another category. We thus first have to create a shared
+**Measure Category** where both units have a conversion relationship.
+
+To do so, go into your sales module select 
+:menuselection:`Configuration --> Products --> Unit of Measure`. 
+Create a new unit of **Measure Category** by selecting
+the dropdown list and clicking on create and edit (see picture below).
+
+.. image:: media/uom03.png
+    :align: center
+
+Create a new unit of measure. In this case our category will be called
+**Inter-Category-Computation**.
+
+.. image:: media/uom06.png
+    :align: center
+
+The next step is to create the **Rolls** and **Square meter** units of
+measure and to link them to the new category. To do so, go into your
+purchase module select :menuselection:`Configuration --> Products --> Units of Measure`.
+
+Create two new units:
+
+-   The **Roll** unit who is part of the Inter-Category-Computation
+    category and is the **Reference Unit type** (see picture below). The
+    Reference Unit type is the measure set as a reference within the
+    category. Meaning that other measures will be converted depending
+    on this measure (ex: 1 roll = 10 square meters, 2 rolls = 20
+    square meters, etc.).
+
+.. image:: media/uom04.png
+    :align: center
+
+-   For the **Square Meter**, we will specify that ``1 Roll = 10 square meters``
+    of curtain. It will thus be necessary to specify that as type,
+    the square meter is bigger than the reference unit. The **Bigger
+    Ratio** is ``10`` as ``one Roll = 10 square meters``.
+
+.. image:: media/uom09.png
+    :align: center
+
+It is now possible to input **square meters** as Unit of measure and a
+**Roll** as Purchase Unit of Measure in the product form.
+
+.. image:: media/uom07.png
+    :align: center
+
+.. seealso::
+    * :doc:`configuration`
+
+.. |uom01| image:: ./media/uom01.png
+.. |uom02| image:: ./media/uom02.png