Add documentation

Author: elbuo8

docs.md

@@ -489,6 +489,7 @@
 | availability_zones | The list of availability zones of the VPC. |
 | internal_rtb_id | The internal route table ID. |
 | external_rtb_id | The external route table ID. |
+| external_nat_ips | A comma-separated list of internal NAT Gateways IPs. |
 
 # web-service
 

vpc/main.tf

@@ -183,6 +183,6 @@ output "external_rtb_id" {
 }
 
 // The list of EIPs associated with the internal subnets.
-output "external_nat_ips" {
+output "internal_nat_ips" {
   value = ["${aws_eip.nat.*.public_ip}"]
 }