Add usage docs for magnum client

Closes-Bug: #1663646
Change-Id: I543a8cc02bd86e6823c82138dfdb054f23b5c250

doc/source/usage.rst

-========
-Usage
-========
+Python bindings to the OpenStack Magnum API
+===========================================
 
-To use python-magnumclient in a project::
+This is a client for the OpenStack Magnum API. It includes a Python
+API (the :mod:`magnumclient` module) and a command-line script
+(installed as :program:`magnum`).
 
-	import magnumclient
\ No newline at end of file
+Python API
+==========
+
+To use python-magnumclient in a project, create a client instance
+using the keystoneauth session API::
+
+    from keystoneauth1.identity import v3
+    from keystoneauth1 import session
+    from keystoneclient.v3 import client
+
+    from magnumclient.client import Client
+
+    magnum_endpoint = "http://magnum.example.com:9511/v1"
+
+    auth = v3.Password(auth_url='http://my.keystone.com:5000/v3',
+                       username='myuser',
+                       password='mypassword',
+                       project_name='myproject',
+                       user_domain_id='default',
+                       project_domain_id='default')
+    sess = session.Session(auth=auth)
+
+    magnum = Client('1', endpoint_override=magnum_endpoint, session=sess)
+    magnum.clusters.list()
+
+For more information on keystoneauth API, see `Using Sessions`_.
+
+.. _Using Sessions: http://docs.openstack.org/developer/keystoneauth/using-sessions.html
+
+Command-line tool
+=================
+
+In order to use the CLI, you must provide your OpenStack username,
+password, project name, user domain ID, project domain ID, and auth
+endpoint. Use the corresponding configuration options (--os-username,
+--os-password, --os-project-name, --os-project-domain-id,
+--os-user-domain-id, and --os-auth-url) or set them in environment
+variables::
+
+    export OS_USERNAME=myuser
+    export OS_PASSWORD=mypassword
+    export OS_PROJECT_NAME=myproject
+    export OS_USER_DOMAIN_ID=default
+    export OS_PROJECT_DOMAIN_ID=default
+    export OS_AUTH_URL=http://my.keystone.com:5000/v3
+
+From there, all shell commands take the form::
+
+    magnum <command> [arguments...]
+
+Run :program:`magnum help` to see a complete listing of available
+commands.  Run :program:`magnum help <command>` to get detailed help
+for that command.