[contributor] Add topic tags explanation

Change-Id: I2d11701adb9f3900a826c8be54c944e75120ab4f
Closes-Bug: #1514373

doc/contributor-guide/source/index.rst

@@ -20,6 +20,7 @@ Contents
    additional-git-workflow.rst
    writing-style.rst
    topic-structure.rst
+   topic-tags.rst
    rst-conv.rst
    docbook-conv.rst
    docs-structure.rst

doc/contributor-guide/source/topic-tags.rst

@@ -0,0 +1,57 @@
+.. _topic_tags:
+
+==========
+Topic tags
+==========
+
+We recommend that you add the tags in the head of the git commit
+message or email subject line to show the topic explicitly.
+Based on the topic your request refers to, use the following tags:
+
+[api]
+  API Guide, API Complete References
+
+[arch-design]
+  Architecture Design Guide
+
+[cli-ref]
+  Command-Line Interface Reference
+
+[common]
+  Common contents for several guides in the common directory
+
+[config-ref]
+  Configuration Reference
+
+[contributor]
+  Documentation Contributor Guide
+
+[firstapp]
+  Writing your first OpenStack application
+
+[ha-guide]
+  High Availability Guide
+
+[image-guide]
+  Virtual Machine Image Guide
+
+[install]
+  Installation guides
+
+[networking]
+  Networking Guide
+
+[ops-guide]
+  Operations Guide
+
+[sec-guide]
+  Security Guide
+
+[training]
+  Training labs, Training guides
+
+[user-guides]
+  End User Guide, Admin User Guide, Cloud Administrator Guide
+
+[www]
+  Web page contents