[readme] Document datadog_site variable (#492)

Author: olivielpeau

README.md

@@ -294,7 +294,8 @@ These variables can be set in the `datadog_agent` class to control settings in t
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 | `collect_ec2_tags`          | Set this to yes to have an instance's custom EC2 tags used as agent tags.                                                                                                                        |
 | `collect_instance_metadata` | Set this to yes to have an instance's EC2 metadata used as agent tags.                                                                                                                           |
-| `dd_url`                    | The Datadog intake server URL. You are unlikely to need to change this.                                                                                                                          |
+| `datadog_site`              | The Datadog site to report to. Defaults to `datadoghq.com`, set to `datadoghq.eu` to report to the EU site. Supported since v2.4.0 of the module, and only with Agent v6+                         |
+| `dd_url`                    | The Datadog intake server URL. You are unlikely to need to change this. Overrides `datadog_site`                                                                                     |
 | `host`                      | Overrides the node's host name.                                                                                                                                                                  |
 | `local_tags`                | An array of <KEY:VALUE> strings that are set as tags for the node.                                                                                                                               |
 | `non_local_traffic`         | Set this to allow other nodes to relay their traffic through this one.                                                                                                                           |