Add documentation for skip conditions

docs/source/releases/v0.8.rst

 
 See :ref:`us_site` for more information.
 
+Checkout improvements
+---------------------
+
+The checkout process now skips payment if the order total is zero (e.g. when
+ordering free products or using a voucher). As part of that, checkout views
+now evaluate *pre-conditions* (as before) and newly introduced
+*skip conditions*. This should make customising the checkout flow easier.
+
 Cleanup around shipping methods
 -------------------------------
 
   ``Product.recommended_products`` to model products that are loosely related
   (e.g. used for upselling). ``Product.related_products`` was a
   third option that sat somewhere in between, and which was not well supported.
-  We fear it adds confusion, and in the spirit of keeping Oscare core lean,
+  We fear it adds confusion, and in the spirit of keeping Oscar core lean,
   has been removed. If you're using it, switch to
   ``Product.recommended_products`` or just add the field back to your
   custom Product instance and ``ProductForm`` when migrating.

oscar/apps/checkout/session.py

     All checkout views subclass this mixin. It ensures that all relevant
     checkout information is available in the template context.
     """
-    # This should be list of method names that get executed before the normal
-    # flow of the view. Each method should check some condition has been met.
-    # If not, then an exception is raised that indicates the URL the customer
-    # should be redirect to.
+
+    # A pre-condition is a condition that MUST be met in order for a view
+    # to be available. If it isn't then the customer should be redirected
+    # to a view *earlier* in the chain.
+    # pre_conditions is a list of method names that get executed before the
+    # normal flow of the view. Each method should check some condition has been
+    # met. If not, then an exception is raised that indicates the URL the
+    # customer will be redirected to.
+
     pre_conditions = None
 
-    # Skip conditions check whether the view should be skipped. They work in
-    # the same way as pre-conditions.
+    # A *skip* condition is a condition that MUST NOT be met in order for a
+    # view to be available. If the condition is met, this means the view MUST
+    # be skipped and the customer should be redirected to a view *later* in
+    # the chain.
+    # Skip conditions work similar to pre-conditions, and get evaluated after
+    # pre-conditions have been evaluated.
     skip_conditions = None
 
     def dispatch(self, request, *args, **kwargs):