update documentation

Signed-off-by: flashdagger <flashdagger@googlemail.com>

docs/index.rst

@@ -11,7 +11,8 @@ an object-oriented Zammad REST API Client for Python
 .. toctree::
    :maxdepth: 2
 
-   intro.rst
+   intro
+   tutorial/index
    contributing
    api/index
    history

docs/tutorial/index.rst

@@ -0,0 +1,8 @@
+========
+Tutorial
+========
+
+.. toctree::
+    :maxdepth: 1
+
+    resources

docs/tutorial/resources.rst

@@ -0,0 +1,28 @@
+Objects can be lazy
+===================
+
+| Consider the following example:
+| We enable logging and create a ticket object
+  by its id. Then we print the ticket id, number and title.
+
+.. code-block:: python
+
+    import logging
+
+    logging.basicConfig(level=logging.INFO)
+
+    ticket = client.tickets(1)
+    print(f"ticket id: {ticket.id}")
+    print(f"ticket number: #{ticket.number}")
+    print(f"ticket title: {ticket.title!r}")
+
+    # ticket id: 1
+    # INFO:zammadoo:HTTP:GET https://localhost/api/v1/tickets/1
+    # ticket number: #67001
+    # ticket title: 'Welcome to Zammad!'
+
+What happened? Before printing the ticket number the client made a server request
+to fetch the ticket data. This means if we instantiate an object it only contains
+its id. The first time we get an attribute or perform a method which requires attributes
+the data is queried from the server. This is called lazy object initialization and applies
+to most resources.