Add Coil CNI to networking.md

Coil is a new open-source CNI plugin for Kubernetes.

Resources:
- https://github.com/cybozu-go/coil
- https://blog.kintone.io/entry/coilv2

content/en/docs/concepts/cluster-administration/networking.md

@@ -158,6 +158,11 @@ tables to provide per-instance subnets to each host (which is limited to 50-100
 entries per VPC). In short, cni-ipvlan-vpc-k8s significantly reduces the
 network complexity required to deploy Kubernetes at scale within AWS.
 
+### Coil
+
+[Coil](https://github.com/cybozu-go/coil) is a CNI plugin designed for ease of integration, providing flexible egress networking.
+Coil operates with a low overhead compared to bare metal, and allows you to define arbitrary egress NAT gateways for external networks.
+
 ### Contiv
 
 [Contiv](https://github.com/contiv/netplugin) provides configurable networking (native l3 using BGP, overlay using vxlan,  classic l2, or Cisco-SDN/ACI) for various use cases. [Contiv](https://contiv.io) is all open sourced.
@@ -311,4 +316,4 @@ to run, and in both cases, the network provides one IP address per pod - as is s
 
 The early design of the networking model and its rationale, and some future
 plans are described in more detail in the
-[networking design document](https://git.k8s.io/community/contributors/design-proposals/network/networking.md).
+[networking design document](https://git.k8s.io/community/contributors/design-proposals/network/networking.md).