README: mention how one could reuse nagios resources with their own logic

Some people might want to inject their own logic before including nagios
resources. We can explain that since the nagios resources are in their
own part of the manifests, they can shortcut the module's automatic
handling of it, and call it manually from their own manifests.

README.md

@@ -49,6 +49,13 @@ monitored.
 Nagios will automatically check the ports defined in `ports`, and the
 hostname specified by `nagios_check_ssh_hostname`.
 
+Note that if you need to use some specific logic to decide whether or not to
+create a nagios service check, you should set $manage_nagios to false, and
+use sshd::nagios from within your own manifests. You'll also need to manually
+specify the port to that define. By default, if the $port parameter is not
+specified, it will use the resource name as the port (e.g. if you call it like
+this: `sshd::nagios { '22': }` )
+
 NOTE: this requires that you are using the shared-nagios puppet module which
 supports the nagios native types via `nagios::service`: