Add slide #17 of upstream training

Change-Id: I31020193f0aa2fb95974c0ab8d2ec88a554bfe48

doc/upstream-training/17-commit-message.rst

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+Commit messages
+===============
+
+- quality of the OpenStack GIT history
+- carrot by alerting people to the benefits
+- anyone doing Gerrit code review can act as the stick
+- review the commit message, not just code
+- Developers read mostly, write occassionally
+
+----
+
+Structural split of changes
+===========================
+
+- Small means quicker & easier
+- Easier to revert
+- Easier to bisect
+- Easier to browse
+
+----
+
+Things to avoid
+===============
+
+- Mixing whitespace changes with functional code changes.
+- Mixing two unrelated functional changes.
+- Sending large new features in a single giant commit.
+
+----
+
+Bad : two independent changes
+=============================
+
+.. image:: ./_assets/17-01-bad-two.png
+
+----
+
+Bad : new feature and refactor
+==============================
+
+.. image:: ./_assets/17-02-bad-new.png
+
+----
+
+Good : new API + new feature
+============================
+
+.. image:: ./_assets/17-03-good.png
+
+----
+
+Do not assume ...
+=================
+
+- the reviewer understands what the original problem was.
+- the reviewer has access to external web services/site.
+- the code is self-evident/self-documenting.
+
+----
+
+Information in commit messages (1/2)
+====================================
+
+- Describe why a change is being made
+- Read the commit message to see if it hints at improved code structure.
+- Ensure sufficient information to decide whether to review.
+
+----
+
+Information in commit messages (2/2)
+====================================
+
+- Describe any limitations of the current code.
+- Do not include patch set-specific comments.
+- The first commit line is the most important.
+
+----
+
+Required external references
+============================
+
+- Change-id
+- Bug
+- blueprint
+
+----
+
+Optional external references
+============================
+
+- DocImpact
+- SecurityImpact
+- UpgradeImpact
+
+----
+
+GIT commit message structure
+============================
+
+.. image:: ./_assets/17-04-commit-message.png
+
+----
+
+Exercise
+========
+
+review each other messages on the draft
+