[ADD] sale: How to give portal access rights to my customers?

https://docs.google.com/document/d/1KQxiFK6U7Qm6Zg4aTdB-GCnhtC_0yelYdAD9ai1f29o/edit

sales.rst

@@ -11,4 +11,5 @@ Sales
    sales/quotation
    sales/invoicing
    sales/products_prices
-   sales/sale_ebay
+   sales/sale_ebay
+   sales/advanced

sales/advanced.rst

@@ -5,3 +5,4 @@ Advanced Topics
 .. toctree::
    :titlesonly:
 
+   advanced/portal

sales/advanced/portal.rst

@@ -1,3 +1,50 @@
 =================================================
 How to give portal access rights to my customers?
-=================================================
+=================================================
+
+What is Portal access/Who is a portal user?
+============================================
+
+A portal access is given to a user who has the necessity to have access
+to Odoo instance, to view certain documents or information in the
+system.
+
+For Example:
+1.  A long term client who needs to view online quotations.
+2.  Accounting consultant who needs to keep track of the company's financials.
+
+A portal user has only read/view access. He or she will not be able to
+edit any document in the system.
+
+How to give portal access to customers?
+=======================================
+
+From Contacts Module
+--------------------
+
+From the main menu, select **Contacts** menu. If the contact is not
+yet created in the system, click on the create button to create
+new contact. Enter details of the contact and click "save".
+
+.. image:: media/portal01.png
+    :align: center
+
+.. image:: media/portal02.png
+    :align: center
+
+Choose a contact, click on the **Action** menu in the top-center of
+the interface and from the drop down.
+
+Select **Portal Access Management**. A pop up window appears.
+
+.. image:: media/portal03.png
+    :align: center
+
+Enter the login **email ID**, check the box under **In Portal** and
+add the content to be included in the email in the text field box below. Click on **Apply** when you're done.
+
+.. image:: media/portal04.png
+    :align: center
+
+An email will be sent to the specified email address, indicating that
+the contact is now a portal user of the respective instance.