Adding git commit message best practices

Based on [1] and [2]

[1]: https://wiki.openstack.org/wiki/GitCommitMessages

[2]: https://docs.openstack.org/upstream-training/workflow-commit-message.html

Story: 2001605

Task: 12097

Change-Id: Ib82ccbcb3a4c1b0e70404b2c367b0dfc60289e87
Signed-off-by: visnyei <andrea.visnyei@nokia.com>

doc/source/common/git.rst

@@ -90,3 +90,89 @@ You can use `Git Immersion <http://gitimmersion.com/lab_02.html>`_ to work
 through tutorials for learning git.
 
 For reference, use the `Git Reference and Cheat Sheet <https://git-scm.com/doc>`_.
+
+Commit Messages
+===============
+
+Commit messsages are the first things a reviewer sees and are used as
+descriptions in the git log. They provide a description of the history of
+changes in a repository. Commit messages cannot be modified once the patch is
+merged.
+
+Format:
+
+- Summary Line
+- Empty line
+- Body
+- Tags
+
+Summary Line
+------------
+
+The summary line briefly describes the patch content. The character limit is 50
+characters. The summary line should not end with a period. If the change is
+not finished at the time of the commit, start the commit message with WIP.
+
+Body
+----
+
+The body contains the explanation of the issue being solved and why it should
+be fixed, the description of the solution, and additional optional information
+on how it improves the code structure, or references to other relevant patches,
+for example. The lines are limited to 72 characters. The body should contain
+all the important information related to the problem, without assuming that the
+reader understands the source of the problem or has access to external sites.
+
+Tags
+----
+
+Tags are references used to link the change to other tools.
+
+The following tags are required:
+
+- The ``Change-id`` line is a unique hash describing the change,
+  which is generated automatically by a Git commit hook. This should not be
+  changed when rebasing a commit following review feedback, since it is used
+  by Gerrit, to track versions of a patch.
+
+The following tags are optional; however, their use is recommended if they are
+applicable to the patch:
+
+- ``Closes-Bug``: #123456789: use ``Closes-Bug`` if the commit is intended to
+  fully fix and close the bug being referenced. Use the Launchpad ID of the bug
+  for the number; Gerrit automatically creates a link to the bug.
+- ``Partial-Bug``: #123456789: use ``Partial-Bug`` if the commit is only a
+  partial fix and more work is needed. Use the Launchpad ID of the bug
+  for the number; Gerrit automatically creates a link to the bug.
+- ``Related-Bug``: #12456789: use 'Related-Bug' if the commit is merely
+  related to the referenced bug. Use the Launchpad ID of the bug
+  for the number; Gerrit automatically creates a link to the bug.
+- ``Partial-Implements``: Use this tag if the change partially implements
+  a Launchpad blueprint. Use the name of the blueprint as an ID.
+- ``Implements``: Use this tag if the change fully implements
+  a Launchpad blueprint. Use the name of the blueprint as an ID.
+- The ``DocImpact`` tag contains a comment about why
+  the change impacts documentation. Put DocImpact on a line by itself.
+  Use this tag to indicate that documentation is either contained in the
+  patch or has documentation impact.
+  When this tag is included in a commit
+  message, Gerrit creates a bug for the project affected by the change for task
+  tracking, or move to the openstack-api-site as needed.
+- The ``APIImpact`` tag contains a comment about why
+  the change impacts a public HTTP API. Put APIImpact on a line by itself.
+  Use this tag to indicate that the patch impacts a public
+  HTTP API. When this tag is included in a commit message,
+  the API_Working_Group can use it to help find relevant reviews.
+- The ``SecurityImpact`` tag is used to indicate that a change has
+  security implications and should be reviewed by the OpenStack Security Group.
+- The ``UpgradeImpact`` tag contains a comment
+  about why the change impacts upgrades. It is used to indicate that a change
+  has upgrade implications for those doing continuous deployment or N to N+1
+  upgrades. Also consider updating the 'Upgrade Notes' section in the release
+  notes for the affected project.
+- The ``Depends-On``: <gerrit-change-url> tag is used to refer to a change
+  the current one depends on. Use the permalink of the change.
+- ``Task``: 1234: the number of the task in Storyboard implemented by the
+  change.
+- ``Story``: 1234567: the number of the story in Storyboard to which the task
+  being implemented belongs.