Allow a client to unsubscribe to channel

This patch allows a client to unsubscribe to a channel. It is used by
example by collectd where we can have different checks. In StackLight we
have a remote collectd that is running and also a collectd daemon. So
there is two collectd processes. By default we subscribe to all channel
according to our roles. But we don't want to subscribe to
collectd.client, only to collectd.remote_client.

Change-Id: I1a6d42abd93b6288740832cfc07fc08ae0559f12
diff --git a/README.rst b/README.rst
index a0fd8f9..10c8d45 100644
--- a/README.rst
+++ b/README.rst
@@ -98,6 +98,24 @@
           local_linux_storage_swap_usage:
             enabled: False
 
+Sensu Client with subscriptions explicitly disabled
+
+.. code-block:: yaml
+
+    sensu:
+      client:
+        enabled: true
+        message_queue:
+          engine: rabbitmq
+          host: rabbitmq
+          port: 5672
+          user: monitor
+          password: pwd
+          virtual_host: '/monitor'
+        unsubscribe:
+          - collectd.client
+          - git.client
+
 Sensu Client with community plugins
 
 .. code-block:: yaml