Add slide #15 of upstream training

Change-Id: Id46a520bc318728fdc5ba0f6e10a38fad016e2f6

doc/upstream-training/15-branching-model.rst

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+nvie inspired
+=============
+
+.. image:: ./_assets/15-01-inspired.png
+
+----
+
+Stable branches
+===============
+
+.. image:: ./_assets/15-02-stable-branches.png
+
+----
+
+Appropriate Fixes
+=================
+
+- risk of regression
+- user visible benefit
+- Self-contained
+- fix is already on master
+
+----
+
+Forbidden
+=========
+
+- New features
+- Changes to the external HTTP APIs
+- Changes to Nova's internal AMQP API
+- Changes to the notification definitions
+- DB schema changes
+- Incompatible config file changes
+
+----
+
+2013.2 branch and tags
+======================
+
+.. image:: ./_assets/15-03-branch-tags.png
+
+----
+
+2013.2.4 preparation
+====================
+
+.. image:: ./_assets/15-04-preparation.png
+
+----
+
+Review
+======
+
+.. image:: ./_assets/15-05-review.png
+
+----
+
+Exercise
+========
+
+checkout the latest stable branch
+