doc: add dynamic referencing

Relates #165

Author: Daniel Matula

docs/usage/advanced.rst

@@ -95,3 +95,13 @@ it is enough to set log level to the desired value.
     logging.basicConfig()
     root_logger = logging.getLogger()
     root_logger.setLevel(logging.DEBUG)
+    
+Dynamically referenced EntitySet
+------------------------------------------------
+
+If you need to work with many Entity Sets the same way or if you just need to pick up the used Entity Set name in run-time, you may find out the ability to get an instance of Entity Set proxy dynamically. Here is an example of how you can print a count of all employees:
+
+.. code-block:: python
+
+    count = getattr(northwind.entity_sets, 'Employees').get_entities().count().execute()
+    print(count)