Added docs for commit message tags

Added some documentation for how and when to use commit message tags
in git commit messages. Also added a link to the new article in the
index for the dev ref.

Change-Id: I5aa4c128cd0e25fb12316de21eed262cca810e08

doc/source/devref/commit_message_tags.rst

@@ -0,0 +1,40 @@
+Using Commit Message Tags in Manila
+===================================
+
+When writing git commit messages for code submissions into manila, it can be
+useful to provide tags in the message for both human consumption as well as
+linking to other external resources, such as Launchpad. Each tag should be
+placed on a separate line. The following tags are used in manila.
+
+- **APIImpact** - Use this tag when the code change modifies a public HTTP
+  API interface. This tag indicates that the patch creates, changes, or deletes
+  a public API interface or changes its behavior. The tag may be followed by a
+  reason beginning on the next line. If you are touching manila's API layer and
+  you are unsure if your change has an impact on the API, use this tag anyway.
+- **Change-id** - This tag is automatically generated by a Gerrit hook and is
+  a unique hash that describes the change. This hash should not be changed
+  when rebasing as it is used by Gerrit to keep track of the change.
+- **Closes-Bug: | Partial-Bug: | Related-Bug:** *<#launchpad_bug_id>* - These
+  tags are used when the change closes, partially closes, or relates to the bug
+  referenced by the Launchpad bug ID respectively. This will automatically
+  generate a link to the bug in Launchpad for easy access for reviewers.
+- **DocImpact** - Use this tag when the code change requires changes or
+  updates to documentation in order to be understood. This tag can also be
+  used if the documentation is provided along with the patch itself. This will
+  also generate a Launchpad bug in manila for triaging and tracking.
+- **Implements: | Partially Implements:** *blueprint <name_of_blueprint>* - Use
+  this tag when a change implements or partially implements the given blueprint
+  in Launchpad. This will automatically generate a link to the blueprint in
+  Gerrit for easy access for reviewers.
+- **TrivialFix** - This tag is used for a trivial issue, such as a typo, an
+  unclear log message, or a simple code refactor that does not change existing
+  behavior which does not require the creation of a separate bug or blueprint
+  in Launchpad.
+
+Make sure that the **Closes-Bug**, **Partial-Bug**, **Related-Bug**,
+**blueprint**, and **Change-id** tags are at the very end of the commit
+message. The Gerrit hooks will automatically put the hash at the end of the
+commit message. For more information on tags and some examples of good commit
+messages, refer to the GitCommitMessages_ documentation.
+
+.. _GitCommitMessages: https://wiki.openstack.org/wiki/GitCommitMessages#Including_external_references

doc/source/devref/index.rst

@@ -30,6 +30,7 @@ Programming HowTos and Tutorials
     unit_tests
     addmethod.openstackapi
     adding_release_notes
+    commit_message_tags
 
 
 Background Concepts for manila