[ADD] purchase: How to analyze the performance of my vendors?

https://docs.google.com/document/d/1tO1wUXSGlqfXbFqmVAex6hFdpQt5XzEvmVFxhVhyx94/edit#heading=h.squnxk7d3pgu

purchase/purchases/rfq/analyze.rst

@@ -1,3 +1,103 @@
 =============================================
 How to analyze the performance of my vendors?
-=============================================
+=============================================
+
+If your company regularly buys products from several suppliers, it would
+be useful to get statistics on your purchases. There are several reasons
+to track and analyze your vendor’s performance :
+
+-  You can see how dependant from a supplier your company is;
+
+-  you can negotiate discounts on prices;
+
+-  You can check the average delivery time per supplier;
+
+-  Etc.
+
+For example, an IT products reseller that issues dozens of purchase
+orders to several suppliers each week may want to measure for each
+product the total price paid for each vendor and the delivery delay. The
+insights gathered by the company will help it to better analyze,
+forecast and plan their future orders.
+
+Configuration
+=============
+
+Install the Purchase Management module
+--------------------------------------
+
+From the **Apps** menu, search and install the **Purchase Management**
+module.
+
+.. image:: media/analyze03.png
+    :align: center
+
+Issue some purchase orders
+--------------------------
+
+Of course, in order to analyze your vendors' performance, you need to
+issue some **Request For Quotations** (RfQ) and confirm some **Purchase
+Orders**. If you want to know how to generate a purchase order, please
+read the documentation :doc:`../../overview/process/from_po_to_invoice`.
+
+Analyzing your vendors
+======================
+
+Generate flexible reports
+-------------------------
+
+You have access to your vendors' performances on the Reports menu.
+By default, the report groups all your purchase orders on a pivot table
+by **total price**, **product quantity** and **average price** for the **each month**
+and for **each supplier**. Simply by accessing this basic report, you can
+get a quick overview of your actual performance. You can add a lot of
+extra data to your report by clicking on the **Measures** icon.
+
+.. image:: media/analyze04.png
+    :align: center
+
+.. tip::
+    By clicking on the **+** and **-** icons, you can drill up and down 
+    your report in order to change the way your information is displayed. 
+    For example, if I want to see all the products bought for the current 
+    month, I need to click on the **+** icon on the vertical axis and 
+    then on “Products”.
+
+
+Depending on the data you want to highlight, you may need to display
+your reports in a more visual view. You can transform your report in
+just a click in 3 graph views : a **Pie Chart**, a **Bar Chart** and a **Line
+Chart**: These views are accessible through the icons highlighted on the
+screenshot below.
+
+.. image:: media/analyze02.png
+    :align: center
+
+.. note:: 
+    On the contrary to the pivot table, a graph can only 
+    be computed with one dependent and one independent measure.
+
+Customize reports
+=================
+
+You can easily customize your purchase reports depending on your needs.
+To do so, use the **Advanced search view** located in the right hand side
+of your screen, by clicking on the magnifying glass icon at the end of
+the search bar button. This function allows you to highlight only
+selected data on your report. The **filters** option is very useful in
+order to display some categories of datas, while the **Group by** option
+improves the readability of your reports. Note that you can filter and
+group by any existing field, making your customization very flexible and
+powerful.
+
+.. image:: media/analyze01.png
+    :align: center
+
+.. tip:: 
+    You can save and reuse any customized filter by clicking 
+    on **Favorites** from the **Advanced search view** and 
+    then on **Save current search**. The saved filter will 
+    then be accessible from the **Favorites** menu.
+
+.. seealso::
+    * :doc:`../../overview/process/from_po_to_invoice`