add example in docs for containerd

Signed-off-by: yanxuean <yan.xuean@zte.com.cn>

docs/alternative_runtimes.md

@@ -30,3 +30,25 @@ $ minikube start \
     --extra-config=kubelet.image-service-endpoint=/var/run/crio/crio.sock \
     --bootstrapper=kubeadm
 ```
+
+### Using containerd
+
+To use [containerd](https://github.com/containerd/containerd) as the container runtime, run:
+
+```shell
+$ minikube start \
+    --network-plugin=cni \
+    --container-runtime=containerd \
+    --bootstrapper=kubeadm
+```
+
+Or you can use the extended version:
+
+```shell
+$ minikube start \
+    --network-plugin=cni \
+    --extra-config=kubelet.container-runtime=remote \
+    --extra-config=kubelet.container-runtime-endpoint=unix:///run/containerd/containerd.sock \
+    --extra-config=kubelet.image-service-endpoint=unix:///run/containerd/containerd.sock \
+    --bootstrapper=kubeadm
+```