NAT protocol is tcpudp not tcp+udp, add note to README

Looks like NAT protocols are named differently to firewall protocols.  Without this change (i.e. using tcp+udp) this error occurs:
```
  networkConfigureEdgeGatewayServices      0/100  |                                                  |  0:00  E, [2014-06-12T11:04:33.445509 #8775] ERROR -- : Could not update EdgeGateway ... : status: error, error: {:minorErrorCode=>"NETWORK_INTERNAL_SERVER", :message=>"Unable to perform this action. Contact your cloud administrator.", :majorErrorCode=>"500"}
/var/lib/gems/1.9.1/gems/fog-1.22.1/lib/fog/vcloud_director/compute.rb:430:in `wait_and_raise_unless_success': status: error, error: {:minorErrorCode=>"NETWORK_INTERNAL_SERVER", :message=>"Unable to perform this action. Contact your cloud administrator.", :majorErrorCode=>"500"} (Fog::Compute::VcloudDirector::TaskError)
	from /var/lib/gems/1.9.1/gems/fog-1.22.1/lib/fog/vcloud_director/compute.rb:419:in `process_task'
	from /var/lib/gems/1.9.1/gems/vcloud-core-0.2.0/lib/vcloud/fog/service_interface.rb:131:in `post_configure_edge_gateway_services'
	from /var/lib/gems/1.9.1/gems/vcloud-core-0.2.0/lib/vcloud/core/edge_gateway.rb:26:in `update_configuration'
	from /var/lib/gems/1.9.1/gems/vcloud-edge_gateway-1.0.0/lib/vcloud/edge_gateway/configure.rb:25:in `update'
	from /var/lib/gems/1.9.1/gems/vcloud-edge_gateway-1.0.0/lib/vcloud/edge_gateway/cli.rb:34:in `run'
	from /var/lib/gems/1.9.1/gems/vcloud-edge_gateway-1.0.0/bin/vcloud-edge-configure:5:in `<top (required)>'
	from /usr/local/bin/vcloud-edge-configure:23:in `load'
	from /usr/local/bin/vcloud-edge-configure:23:in `<main>'
```