Add order model example to docs

Currently the "Checkout and Payment" documentation page has an example payment.py that does not work if you follow the guide, because the guide never has you set up the custom Order model that it's using behind the scenes.

docs/checkout_and_payment.rst

@@ -13,7 +13,15 @@ which accepts a list of dotted paths to :class:`salesman.checkout.payment.Paymen
 
 .. raw:: html
 
-    <h3>1. Create payment method</h3>
+    <h3>1. Create order model</h3>
+
+In this example we don't modify the Order model, but you can if you want to.
+
+.. literalinclude:: /../example/shop/models/order.py
+
+.. raw:: html
+
+    <h3>2. Create payment method</h3>
 
 First create your custom payment method. Payment methods are required to specify a ``label`` and
 a unique ``identifier`` property on class. To enable payment for the basket you should also
@@ -23,7 +31,7 @@ override the :meth:`salesman.checkout.payment.PaymentMethod.basket_payment` meth
 
 .. raw:: html
 
-    <h3>2. Register payment method</h3>
+    <h3>3. Register payment method</h3>
 
 Then register your payment method in ``settings.py``: