[ADD] purchase: Manage Call for Tender

https://docs.google.com/document/d/1s-OT7SDAGdGeKEWIW--EF9LTW6RBiPI7QQXgGcvhooU/edit

purchase/purchases/tender.rst

@@ -8,3 +8,4 @@ Purchase Agreements
    tender/manage_multiple_offers
    tender/partial_purchase
    tender/blanket_orders
+   tender/call_for_tender

purchase/purchases/tender/call_for_tender.rst

@@ -0,0 +1,55 @@
+======================
+Manage Call for Tender
+======================
+
+A Call for Tender is a special procedure to request offers from multiple
+vendors to obtain the most interesting price.
+
+Configuration
+=============
+
+For this feature to work, go to :menuselection:`Purchases -->
+Configuration --> Settings` and activate the *Purchase Agreements*
+feature.
+
+.. image:: media/call_for_tender01.png
+    :align: center
+
+Create a Call for Tender
+========================
+
+To use this feature go to :menuselection:`Purchases --> Purchase Agreements`.
+
+Create a new purchase agreement, access the *Agreement Type* drop down
+menu and select *Call for Tender*.
+
+When you are satisfied with your purchase agreement, confirm it will
+move from *Draft* to *Confirmed* and a new *RFQs/Orders* appeared
+in the top right corner of the document.
+
+.. image:: media/call_for_tender02.png
+    :align: center
+
+Request new quotations from the Call for Tender
+-----------------------------------------------
+
+From the Call for Tender, Odoo will auto-fill the RFQ with the product(s)
+from your Call for Tender.
+
+Select the best offer
+=====================
+
+The various RFQs and orders linked to the Call for Tenders will be under
+the *RFQs/Orders* button where you can select and confirm the best
+offer.
+
+.. image:: media/call_for_tender03.png
+    :align: center
+
+Close the Call for Tender
+-------------------------
+
+Once you're done with your *Call for Tender* don't forget to close it.
+
+.. image:: media/call_for_tender04.png
+    :align: center