Add unifi_setting_usg

Fixes #183

docs/resources/setting_usg.md

@@ -0,0 +1,31 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "unifi_setting_usg Resource - terraform-provider-unifi"
+subcategory: ""
+description: |-
+  unifi_setting_usg manages settings for a Unifi Security Gateway.
+---
+
+# unifi_setting_usg (Resource)
+
+`unifi_setting_usg` manages settings for a Unifi Security Gateway.
+
+
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Optional
+
+- **dhcp_relay_servers** (List of String) The DHCP relay servers.
+- **firewall_guest_default_log** (Boolean) Whether the guest firewall log is enabled.
+- **firewall_lan_default_log** (Boolean) Whether the LAN firewall log is enabled.
+- **firewall_wan_default_log** (Boolean) Whether the WAN firewall log is enabled.
+- **multicast_dns_enabled** (Boolean) Whether multicast DNS is enabled.
+- **site** (String) The name of the site to associate the settings with.
+
+### Read-Only
+
+- **id** (String) The ID of the settings.
+
+