[ADD] sale: From quotation to sales order

https://goo.gl/ZxJWfI

sales/overview/main_concepts/invoicing.rst

@@ -1,3 +1,100 @@
 =================================
 Overview of the invoicing process
-=================================
+=================================
+
+Depending on your business and the application you use, there are
+different ways to automate the customer invoice creation in Odoo.
+Usually, draft invoices are created by the system (with information
+coming from other documents like sales order or contracts) and
+accountant just have to validate draft invoices and send the invoices in
+batch (by regular mail or email).
+
+Depending on your business, you may opt for one of the following way to
+create draft invoices:
+
+:menuselection:`Sales Order --> Invoice`
+----------------------------------------
+
+In most companies, salespeople create quotations that become sales order
+once they are validated. Then, draft invoices are created based on the
+sales order. You have different options like:
+
+-  Invoice on ordered quantity: invoice the full order before
+   triggering the delivery order
+
+-  Invoice based on delivered quantity: see next section
+
+Invoice before delivery is usually used by the eCommerce application
+when the customer pays at the order and we deliver afterwards.
+(pre-paid)
+
+For most other use cases, it's recommended to invoice manually. It
+allows the salesperson to trigger the invoice on demand with options:
+invoice ready to invoice line, invoice a percentage (advance), invoice a
+fixed advance.
+
+This process is good for both services and physical products.
+
+Read more: *Invoice based on sales orders.*
+
+:menuselection:`Sales Order --> Delivery --> Invoice`
+-----------------------------------------------------
+
+Retailers and eCommerce usually invoice based on delivered quantity ,
+instead of sales order. This approach is suitable for businesses where
+the quantities you deliver may differs from the ordered quantities:
+foods (invoice based on actual Kg).
+
+This way, if you deliver a partial order, you only invoice for what you
+really delivered. If you do back orders (deliver partially and the rest
+later), the customer will receive two invoices, one for each delivery
+order.
+
+.. todo::
+    Read more: *Invoice based on delivery orders.*
+
+:menuselection:`Recurring Contracts (subscriptions) --> Invoices`
+-----------------------------------------------------------------
+
+For subscriptions, an invoice is triggered periodically, automatically.
+The frequency of the invoicing and the services/products invoiced are
+defined on the contract.
+
+.. todo::
+    Read more: *Subscription based invoicing.*
+
+:menuselection:`eCommerce Order --> Invoice`
+--------------------------------------------
+
+An eCommerce order will also trigger the creation of the order when it
+is fully paid. If you allow paying orders by check or wire transfer,
+Odoo only creates an order and the invoice will be triggered once the
+payment is received.
+
+Creating an invoice manually
+----------------------------
+
+Users can also create invoices manually without using contracts or a
+sales order. It's a recommended approach if you do not need to manage
+the sales process (quotations), or the delivery of the products or
+services.
+
+Even if you generate the invoice from a sales order, you may need to
+create invoices manually in exceptional use cases:
+
+-  if you need to create a refund
+
+-  If you need to give a discount
+
+-  if you need to change an invoice created from a sales order
+
+-  if you need to invoice something not related to your core business
+
+Others
+------
+
+Some specific modules are also able to generate draft invoices:
+
+-  membership: invoice your members every year
+
+-  repairs: invoice your after-sale services