README: add mention to custom_cni (kubernetes-sigs#9878)

README.md

@@ -240,6 +240,9 @@ You can choose among ten network plugins. (default: `calico`, except Vagrant use
 
 - [multus](docs/multus.md): Multus is a meta CNI plugin that provides multiple network interface support to pods. For each interface Multus delegates CNI calls to secondary CNI plugins such as Calico, macvlan, etc.
 
+- [custom_cni](roles/network-plugin/custom_cni/) : You can specify some manifests that will be applied to the clusters to bring you own CNI and use non-supported ones by Kubespray.
+  See `tests/files/custom_cni/README.md` and `tests/files/custom_cni/values.yaml`for an example with a CNI provided by a Helm Chart.
+
 The network plugin to use is defined by the variable `kube_network_plugin`. There is also an
 option to leverage built-in cloud provider networking instead.
 See also [Network checker](docs/netcheck.md).