docs: add gateway qos doc

Makefile

@@ -62,6 +62,7 @@ kind-init:
 	kind delete cluster --name=kube-ovn
 	kind create cluster --config yamls/kind-single.yaml --name kube-ovn
 	kind load docker-image --name kube-ovn ${REGISTRY}/kube-ovn:${RELEASE_TAG}
+	kubectl taint node kube-ovn-control-plane node-role.kubernetes.io/master:NoSchedule-
 	bash dist/images/install.sh
 
 kind-init-ha:

docs/qos.md

@@ -1,7 +1,8 @@
 # QoS
 
-Kube-OVN supports dynamically configurations of Ingress and Egress traffic rate limiting.
+Kube-OVN supports dynamically configurations of Ingress and Egress traffic rate limiting for a single pod level and gateway level.
 
+## Pod QoS
 Use the following annotations to specify QoS:
 - `ovn.kubernetes.io/ingress_rate`: Rate limit for Ingress traffic, unit: Mbit/s
 - `ovn.kubernetes.io/egress_rate`: Rate limit for Egress traffic, unit: Mbit/s
@@ -22,3 +23,25 @@ spec:
   - name: qos
     image: nginx:alpine
 ```
+
+## Gateway QoS
+*This feature is still in master branch*
+
+Kube-OVN will create an `ovn0` interface on each host to route traffic from cluster pod network 
+to external network. Kube-OVN control gateway QoS by modify the QoS config of `ovn0` interface.
+
+For a subnet with central gateway mode, only one node act as the gateway, so you can modify the 
+node QoS annotation to control the QoS of the subnet to external network.
+
+```bash
+apiVersion: v1
+kind: Node
+metadata:
+  annotations:
+    ovn.kubernetes.io/ingress_rate: "3"
+    ovn.kubernetes.io/egress_rate: "1"
+  name: liumengxin-ovn1-192.168.16.44
+```
+
+You can also use this annotation to control the traffic from each node to external network 
+through these annotations.