doc: and actually add the section updating

Author: filak-sap

docs/usage/updating.rst

@@ -0,0 +1,36 @@
+Updating
+========
+
+To update an entity, you must create an updated request, set properties to the
+values you want and execute the request. The library will send an HTTP PATCH
+request to the remote service.
+
+.. code-block:: python
+
+    import pyodata
+    import requests
+
+    SERVICE_URL = 'http://services.odata.org/V2/Northwind/Northwind.svc/'
+
+    northwind = pyodata.Client(SERVICE_URL, requests.Session())
+
+    update_request = northwind.entity_sets.Customers.update_entity(CustomerID='ALFKI')
+    update_request.set(CompanyName='Alfons Kitten')
+    update_request.execute()
+
+
+In the case the service you are dealing with requires PUT method, you have two options.
+
+The first option allows you to change the used HTTP method for a single call via
+the key word parameter *method* of the method *update_entity*.
+
+... code-block:: python
+
+    update_request = northwind.entity_sets.Customers.update_entity(CustomerID='ALFKI', method='PUT')
+
+If you need to run more update requests for different entity sets and all of them must be *PUT*,
+then you can consider setting the default service's update method to *PUT*.
+
+... code-block:: python
+
+    northwind.config['http']['update_method'] = 'PUT'