Add some documentation on enabling logging

This patch adds documentation on how to enable logging.
Related-Bug: #1521265

Change-Id: Ib0a84bcf1af12a5b489cee8abcb4231f8e34151e

doc/source/usage.rst

@@ -1,6 +1,6 @@
-========
+=====
 Usage
-========
+=====
 
 This document describes some of the common usage patterns for Castellan. When
 incorporating this package into your applications, care should be taken to
@@ -164,6 +164,24 @@ configuration.**
     options.set_defaults(cfg.CONF, api_class='some.other.KeyManager')
     manager = key_manager.API()
 
+Logging from within Castellan
+-----------------------------
+
+Castellan uses ``oslo_log`` for logging. Log information will be generated
+if your application has configured the ``oslo_log`` module. If your
+application does not use ``oslo_log`` then you can enable default logging
+using ``enable_logging`` in the ``castellan.options`` module.
+
+**Example. Enabling default logging.**
+
+.. code:: python
+
+    from castellan import options
+    from castellan import key_manager
+
+    options.enable_logging()
+    manager = key_manager.API()
+
 Generating sample configuration files
 -------------------------------------