Add doc for logger usage

This patch adds a doc to record rules of usage of
loggers in OSC.

Change-Id: I27a09f9abb523393031560e2310bfdd1af0b8922
Implements: blueprint log-usage

doc/source/command-logs.rst

@@ -0,0 +1,73 @@
+============
+Command Logs
+============
+
+Logger usage in OpenStackClient is not exactly the same as those in other
+OpenStack projects. The following basic rules should be followed.
+
+1. OpenStackClient uses python standard logging library instead of oslo.log
+   so that it will depend on oslo as little as possible.
+
+2. All logs except debug log need to be translated. The log message strings
+   that need to be translated should follow the rule of i18n guidelines:
+   http://docs.openstack.org/developer/oslo.i18n/guidelines.html
+
+3. There are mainly two kinds of logs in OpenStackClient: command specific
+   log and general log. Use different logger to record them. The examples
+   below will show the detail.
+
+Command specific log
+====================
+
+Command specific logs are those messages that used to record info, warning
+and error generated from a specific command. OpenStackClient uses the logger
+of the module the command belongs to to record the command specific logs.
+
+Example
+~~~~~~~
+
+This example shows how to log command specific logs in OpenStackClient.
+
+.. code-block:: python
+
+    import logging
+
+    from openstackclient.i18n import _
+
+
+    LOG = logging.getLogger(__name__)     # Get the logger of this module
+
+    ## ...
+
+        LOG.error(_("Error message"))
+        LOG.warning(_("Warning message"))
+        LOG.info(_("Info message"))
+        LOG.debug("Debug message")        # Debug messages do not need to be translated
+
+    ## ...
+
+General log
+===========
+
+General logs are those messages that not specific to any single command. Use
+the logger of ``openstackclient.shell`` to record them. In each command class,
+we can simply get this logger by ``self.app.log``.
+
+Example
+~~~~~~~
+
+This example shows how to log general logs in OpenStackClient.
+
+.. code-block:: python
+
+    from openstackclient.i18n import _
+
+
+    ## ...
+
+        self.app.log.error(_("Error message"))
+        self.app.log.warning(_("Warning message"))
+        self.app.log.info(_("Info message"))
+        self.app.log.debug("Debug message")        # Debug messages do not need to be translated
+
+    ## ...

doc/source/index.rst

@@ -51,6 +51,7 @@ Developer Documentation
    command-options
    command-wrappers
    command-errors
+   command-logs
    specs/commands
 
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