README: clearify how to configure an arbiter node for a replica set

Author: pecharmin

README.md

@@ -420,14 +420,19 @@ set name as an argument to this set. All hosts must have the same set name.
 An array of member hosts for the replica set.
 Mutually exclusive with `replset_config` param.
 
+##### `replset_arbiter`
+A string to identify an arbiter instance for the replica set.
+The arbiter node must also appear in `replset_members`.
+Mutually exclusive with `replset_config` param.
+
 ##### `replset_config`
 A hash that is used to configure the replica set.
 Mutually exclusive with `replset_members` param.
 
 ```puppet
 class mongodb::server {
   replset        => 'rsmain',
-  replset_config => { 'rsmain' => { ensure  => present, members => ['host1:27017', 'host2:27017', 'host3:27017']  }  }
+  replset_config => { 'rsmain' => { ensure  => present, members => ['host1:27017', 'host2:27017', 'host3:27017'], arbiter => ['host3:27017'] } }
 
 }
 ```