[ADD] purchase: How to setup a multi-company sale/purchase flow?

https://docs.google.com/document/d/1EK-tVxbt6W7Uj-nVkhat0rKGKc50x8J3MumdCp4EGlY/edit?usp=sharing

purchase/replenishment/multicompany/setup.rst

@@ -2,4 +2,55 @@
 How to setup a multi-company sale/purchase flow?
 ================================================
 
-*Coming soon...*
+Overview
+========
+
+Odoo is an outstanding solution to help small companies growing their
+business. But it also perfectly meets the needs of multinational
+companies.The inter-company feature helps you to buy and/or sell
+products and services between different branches within your
+conglomerate.
+
+.. image:: media/setup01.png
+    :align: center
+
+1.  Purchase orders and sales orders can be related. If a company within
+    your group creates a purchase or a sales order, the corresponding
+    document is automatically created for your company. All you
+    have to do is check that everything is correct and confirm the
+    sale. You can automate the validation on your sales and purchase
+    orders.
+
+2.  It is also possible to only handle invoices and refunds.
+
+Manage intercompany rules
+=========================
+
+Go to :menuselection:`Settings --> General Settings`. 
+Flag **Manage multiple companies** and then **Manage Inter Company**. 
+
+Click on **Apply**.
+
+.. image:: media/setup02.png
+    :align: center
+
+New options will appear.
+
+.. image:: media/setup03.png
+    :align: center
+
+In the drop-down list, choose the company on which you want to add
+rules.
+
+If you click on **SO and PO setting for inter company**, you will get
+extra options.
+
+.. image:: media/setup04.png
+    :align: center
+
+When you are done, click on **Apply**, then you can repeat the same steps
+for the other companies.
+
+.. note::
+    In order to be able to manage the inter-company rules, be sure
+    that your user has the rights to manage the companies.