docs: describe how to deploy systemd templates

I've bent over backwards to make this work, while the solution is much
simpler than I thought it was.

Closes: #510
Replaces: #511

README.md

@@ -58,6 +58,22 @@ systemd::unit_file { 'foo.service':
 }
 ```
 
+If you're using "template" units (those with a `@` in it), you can
+simply enable it with a `service` resource. This, for example, will
+enable the `foo@bar` service unit, based on the `foo@` template:
+
+```puppet
+systemd::unit_file { '[email protected]':
+  content => file("${module_name}/[email protected]"),
+  enable  => false,
+  active  => false,
+}
+service { '[email protected]':
+  ensure => true,
+  enable => true,
+}
+```
+
 ### unit files from parameters
 
 Create a unit file from parameters