add doc for gateway pod in default vpc

docs/default-vpc-gateway.md

@@ -0,0 +1,103 @@
+
+Kube-OVN has supported the implementation of VPC. The detailed configuration can be referred to 
+[Vpc](https://github.com/kubeovn/kube-ovn/blob/master/docs/vpc.md).
+
+A customized VPC can connect the Internet through the vpc-nat-gw pod. The default vpc can also be configured with a gateway, so that pod in customized vpc can visit the pod in default vpc.
+
+The precondition should be matched in the environment:
+1. The multus-cni and macvlan cni should be installed, which is used for creating an attachment network for the gateway pod.
+2. The configmap of `ovn-vpc-nat-gw-config` in the kube-system namespace should have existed. It should be created if the configmap does not exist, which name is stable.
+
+One detailed configuration for the configmap is as follows:
+
+```yaml
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: ovn-vpc-nat-gw-config
+  namespace: kube-system
+data:
+  image: 'kubeovn/vpc-nat-gateway:v1.9.0'  # Docker image for vpc nat gateway
+  enable-vpc-nat-gw: true                  # 'true' for enable, 'false' for disable
+  nic: eth1                                # The nic that connect to underlay network, use as the 'master' for macvlan
+```
+
+# Configuration for Default Vpc Gateway
+## Create Subnet
+After the installation of Kube-OVN, the default vpc 
+`ovn-cluster` and the default subnet `ovn-default` had been created. The gateway pod for default vpc can be assigned an ip address from the default subnet, and also can be assigned an ip address from a new subnet.
+
+It's an example to assign IP from a new subnet for the gateway pod.
+
+Create subnet with yaml as follows
+
+```
+apiVersion: kubeovn.io/v1
+kind: Subnet
+metadata:
+  name: test
+spec:
+  cidrBlock: 192.100.0.0/16
+  default: false
+  disableGatewayCheck: false
+  disableInterConnection: true
+  gatewayNode: ""
+  gatewayType: distributed
+  natOutgoing: false
+  private: false
+  protocol: IPv4
+  provider: ovn
+  vpc: ovn-cluster
+```
+
+## Create Gateway Pod
+Create a vpc-nat-gw crd with yaml as follows, which will create a gateway pod for the default vpc.
+
+```
+apiVersion: kubeovn.io/v1
+kind: VpcNatGateway
+metadata:
+  name: default
+spec:
+  vpc: ovn-cluster                            # default vpc
+  subnet: test                                # assign ip from the subnet for gateway pod
+  lanIp: 192.100.10.10                        # ip address for gateway pod
+
+  eips:
+  - eipCIDR: 172.18.0.12/16                   # eip address for pod in the default vpc
+    gateway: 172.18.0.1
+  - eipCIDR: 172.18.0.22/16
+    gateway: 172.18.0.1
+```
+
+After the vpc-nat-gw crd is created, a gateway pod will be created automatically in the kube-system namespace.
+
+```
+apple@appledeMacBook-Pro ovn-test % kubectl get pod -n kube-system
+NAME                                             READY   STATUS    RESTARTS   AGE
+coredns-f9fd979d6-dcppf                          1/1     Running   0          4d18h
+coredns-f9fd979d6-fg7rw                          1/1     Running   0          4d18h
+etcd-kube-ovn-control-plane                      1/1     Running   0          4d18h
+kube-apiserver-kube-ovn-control-plane            1/1     Running   0          4d18h
+kube-controller-manager-kube-ovn-control-plane   1/1     Running   0          4d18h
+kube-multus-ds-g782g                             1/1     Running   0          22h
+kube-multus-ds-knj7m                             1/1     Running   0          22h
+kube-ovn-cni-2q6b9                               1/1     Running   0          4d18h
+kube-ovn-cni-6x7jl                               1/1     Running   0          4d18h
+kube-ovn-controller-7658c87bd-kdwd8              1/1     Running   0          4d18h
+kube-ovn-monitor-5dc58b495c-xv5vz                1/1     Running   0          4d18h
+kube-ovn-pinger-9mc6l                            1/1     Running   0          4d18h
+kube-ovn-pinger-xckxs                            1/1     Running   0          4d18h
+kube-proxy-7xk9j                                 1/1     Running   0          4d18h
+kube-proxy-h9r6x                                 1/1     Running   0          4d18h
+kube-scheduler-kube-ovn-control-plane            1/1     Running   0          4d18h
+ovn-central-6b87fcd545-pt8hr                     1/1     Running   0          4d18h
+ovs-ovn-8nvj8                                    1/1     Running   0          4d18h
+ovs-ovn-wffd2                                    1/1     Running   0          4d18h
+vpc-nat-gw-default-cb7b9677f-q6sbg               1/1     Running   0          17h
+apple@appledeMacBook-Pro ovn-test %
+```
+
+There's no need to add a custom route to the gateway pod in the default vpc. The subnet in the default vpc has added route when created.
+
+To visit the pod in the default vpc, it's visiting the eip address configured in gateway pod in fact, which is the same as visit pod in customized vpc.