Change doc (#1)

* Update README.md

* Added diagram.svg

* Update README.md

Author: int128

README.md

@@ -1,18 +1,82 @@
 # terraform-aws-nat-instance
 
-This is a Terraform module which provisions a NAT instance using an auto scaling group and spot request.
+This is a Terraform module to provision a NAT instance for private subnet(s).
+It provides the following features:
+
+- Auto healing using the ASG
+- Lower cost using a spot instance
+- Fixed public IP address using an EIP and ENI
+- SSM session manager support
+
+Take a look at the diagram:
+
+![diagram](diagram.svg)
+
+Note that you should use a NAT gateway in general.
+This module is only for development or testing purpose.
+
+
+## Getting Started
+
+```tf
+module "vpc" {
+  source  = "terraform-aws-modules/vpc/aws"
+
+  name                 = "hello-vpc"
+  cidr                 = "172.18.0.0/16"
+  private_subnets      = ["172.18.64.0/20", "172.18.80.0/20", "172.18.96.0/20"]
+  public_subnets       = ["172.18.128.0/20", "172.18.144.0/20", "172.18.160.0/20"]
+  enable_dns_hostnames = true
+}
+
+module "nat" {
+  source = "github.com/int128/terraform-aws-nat-instance"
+
+  name                        = "hello-nat"
+  vpc_id                      = module.vpc.vpc_id
+  public_subnet               = module.vpc.public_subnets[0]
+  private_subnets_cidr_blocks = module.vpc.private_subnets_cidr_blocks
+
+  # (Optional)
+  # you can specify this to set the default route to the ENI in the route tables
+  private_route_table_ids = module.vpc.private_route_table_ids
+}
+```
 
 
 ## How it works
 
-This provisions an EC2 instance for NAT.
+This module provisions the following resources:
+
+- Launch Template
+- Auto Scaling Group with miexed instances policy
+- Elastic IP
+- Elastic Network Interface
+- Security Group (allow from private subnets and to Internet)
+- IAM Role for SSM and ENI attachment
+- VPC Route (optional)
+
+The auto scaling group will create an instance.
 
 The instance does the following things on startup:
 
 1. Attach the ENI to `eth1`.
 1. Enable IP forwarding.
 1. Set to ignore ICMP redirect packets.
 1. Enable IP masquerade.
-1. Tear down `eth0`.
+1. Switch the default route to `eth1`.
 
 See [init.sh](data/init.sh) for more.
+
+
+## TODOs
+
+- [ ] Outputs
+- [ ] Variables descriptions
+- [ ] CI
+- [ ] Parameters list in README.md
+
+
+## Contributions
+
+This is an open source software. Feel free to open issues and pull requests.