[ADD] purchase: From purchase order to invoice and receptions

https://docs.google.com/document/d/1FQ109VYZn6Q4MrAykVws17jd1ofwQWW9jUAxyilcy7Q/edit#

purchase/overview/process/from_po_to_invoice.rst

@@ -2,4 +2,125 @@
 From purchase order to invoice and receptions
 =============================================
 
-*Coming soon...*
+For most of your everyday purchases, chances are you already know where
+to purchase and at what price. For these cases, a simple Purchase Order
+(PO) will allow you to handle the whole process.
+
+In Odoo, a purchase order can be created as is, but can also be the
+result of a Request for Quotation or of a Purchase Tender. Therefore,
+every purchase made in Odoo has a PO.
+
+The PO will generate an invoice, and depending on the contract with your
+supplier, you will be required to pay the invoice before or after
+delivery.
+
+Install the Purchase Management application
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+From the **Apps** application, search and install the 
+**Purchase Management** application.
+
+.. image:: media/from_po_to_invoice01.png
+    :align: center
+
+Creating a Purchase Order
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the **Purchases** app, open the **Purchase** menu and 
+click on **Purchase Orders**.
+
+In the **Purchase Orders** window, click on **Create**.
+
+.. image:: media/from_po_to_invoice02.png
+    :align: center
+
+From the new window, insert the **Vendor** and type in the **Order
+Date**.
+
+In the **Products** section, click on **Add an item**.
+
+.. image:: media/from_po_to_invoice03.png
+    :align: center
+
+Select your product from the list and add a description if necessary.
+
+The **Scheduled Date** menu corresponds to the expected delivery date.
+
+Type in the quantity which you wish to purchase, then click on **Save** and
+on **Confirm Order**.
+
+.. note::
+    If you wish to go through the complete flow and create a **Request for 
+    Quotation** for this order, refer to the document on 
+    :doc:`../../purchases/rfq/create`
+
+As you can see, the status of the PO has switched to ``Purchase Order``.
+
+.. image:: media/from_po_to_invoice04.png
+    :align: center
+
+Registering invoice, payments and receiving products
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Depending on the contract you have with your supplier, you can either
+pay for the purchase upon delivery of the goods, or get the goods
+delivered after payment.
+
+Payment upon or after reception
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Still from your purchase order, click on **Receive Products**.
+
+In the next page, check that the number of products received corresponds
+to the number ordered, then manually enter the delivered quantity and
+click on **Validate**.
+
+.. image:: media/from_po_to_invoice05.png
+    :align: center
+
+Go back to the PO. In the PO, a **Shipment** and an **Invoice** button have
+appeared.
+
+.. image:: media/from_po_to_invoice06.png
+    :align: center
+
+Click on the **Invoices** button, then click on **Validate**. 
+The invoice is now registered in the system. Click on **Register Payment**, 
+insert the detail of the payment, and click on **Validate**.
+
+.. image:: media/from_po_to_invoice07.png
+    :align: center
+
+Your products are now ready for picking and storage, and the invoice is
+marked as paid.
+
+Upfront payment
+^^^^^^^^^^^^^^^
+
+From the **Purchase Order** page, open the **Invoices** tab, then click on
+**Create**.
+
+In the next page, click on **Validate**. The invoice is now registered in
+the system. Click on **Register Payment**, insert the detail of the payment,
+and click on **Validate**.
+
+.. image:: media/from_po_to_invoice08.png
+    :align: center
+
+Go back to the PO. In the PO, a **Shipment** tab and an **Invoice** tab have
+appeared.
+
+.. image:: media/from_po_to_invoice09.png
+    :align: center
+
+Click on **Receive Products**, then in the new page, click on **Validate**.
+
+.. image:: media/from_po_to_invoice10.png
+    :align: center
+
+A window will appear, asking if you wish to process every item at once.
+Click on **Apply**.
+
+Your products are now ready for picking and storage, and the invoice is
+marked as paid.
+