[upstream] Add slides on 'WIP' patches to Workflow

It was discussed after the class in Boston that we might want
to add something about 'Work In Progress' patches in the
workflow section somewhere.  After looking through the content the
best place to put it seemed to be in the 'Reviewing' section as
that allowed to tell how a 'WIP' could be indicated and then how
that should be handled from a review standpoint.

This patch adds a couple of slides to cover the topic.

Change-Id: Icf77b248e4cfddf1d6efd5fa1d8b0a94fefdc76d

doc/upstream-training/source/slides/workflow-reviewing.rst

@@ -46,3 +46,23 @@ http://docs.openstack.org/upstream-training/workflow-using-sandbox.html
 Try to find things to make comments on even if they
 are just asking a question and not pointing out an
 issue, don't just +1 three different patches.
+
+Work In Progress Patches
+========================
+
+.. image:: ./_assets/workflow-reviewing-wip-patch.png
+
+Work In Progress Patches
+========================
+
+- Multiple ways a patch can be indicated as a Work In Progress
+
+  - 'Workflow' is set to '-1'
+  - '[WIP]' is at the beginning of the patch's summary line
+
+- Developers often do this to share an early preview of what they are
+  doing or to allow for feedback during development
+- Patches may be reviewed but should be done so with the fact that
+  the patch is still being developed in mind
+
+  - Don't -1 for spelling mistakes, wording or missing details