iSulad: add docs for integration

Signed-off-by: Nleizhongkai <leizhongkai@huawei.com>

README.md

@@ -59,6 +59,9 @@ $ sudo isula rm test
 ### Build from source
 Build requirements for developers are listed in [build_guide](./docs/build_guide.md)
 
+### Integration
+Integrate with `kubenetes` are listed in [integration.md](./docs/integration.md)
+
 ## How to Contribute
 
 We always welcome new contributors. And we are happy to provide guidance for the new contributors.

docs/integration.md

+# Integrate kubenetes
+
+## Configuration
+
+1. Configure `isulad`
+
+   Configure the `pod-sandbox-image`  in `/etc/isulad/daemon.json`:
+
+   ```json
+   "pod-sandbox-image": "my-pause:1.0.0"
+   ```
+
+   Configure the `endpoint`of `isulad`:
+
+   ```json
+   "hosts" : [
+           "unix:///var/run/isulad.sock"
+       ]
+   ```
+
+   if `hosts` is not configured, the default endpoint is `unix:///var/run/isulad.sock`.
+
+2. Restart `isulad`:
+
+   ```bash
+   $ sudo systemctl restart isulad
+   ```
+
+3. Start `kubelet` based on the configuration or default value:
+
+   ```bash
+   $ /usr/bin/bubelet 
+   --container-runtime-endpoint=unix:///var/run/isulad.sock
+   --image-service-endpoint=unix:///var/run/isulad.sock 
+   --pod-infra-container-image=my-pause:1.0.0
+   ...
+   ```
+
+## Use  RuntimeClass
+
+RuntimeClass is used for selecting the container runtime configuration to use to run a pod’s containers, see [runtime-class](https://kubernetes.io/docs/concepts/containers/runtime-class/). Currently, only `kata-containers` and `runc`  this two `oci runtime` are supported.
+
+1. Configure `isulad` in `/etc/isulad/daemon.json`:
+
+   ```json
+   "runtimes": {
+           "runc":{
+               "path": "/usr/bin/runc",
+               "runtime-args": []
+           },
+           "kata-runtime": {
+               "path": "/usr/bin/kata-runtime",
+               "runtime-args": [
+                   "--kata-config",
+                   "/usr/share/defaults/kata-containers/configuration.toml"
+               ]
+           }
+       }
+   ```
+
+2. Extra configuration
+
+   `iSulad` supports the `overlay2` and `devicemapper` as storage drivers. The default value is `overlay2`.
+
+   In some scenarios, using block device type as storage drivers is a better choice, such as run a `kata-containers`. The procedure for configuring the `devicemapper` is as follows:
+
+   Create ThinPool:
+
+   ```bash
+   $ sudo pvcreate /dev/sdb1 # /dev/sdb1 for example
+   $ sudo vgcreate isulad /dev/sdb
+   $ sudo echo y | lvcreate --wipesignatures y -n thinpool isulad -L 200G
+   $ sudo echo y | lvcreate --wipesignatures y -n thinpoolmeta isulad -L 20G
+   $ sudo lvconvert -y --zero n -c 512K --thinpool isulad/thinpool --poolmetadata isulad/thinpoolmeta
+   $ sudo lvchange --metadataprofile isulad-thinpool isulad/thinpool
+   ```
+
+   Add configuration for `devicemapper` in `/etc/isulad/daemon.json`:
+
+   ```json
+   "storage-driver": "devicemapper"
+   "storage-opts": [
+   		"dm.thinpooldev=/dev/mapper/isulad-thinpool",
+   	    "dm.fs=ext4",
+   	    "dm.min_free_space=10%"
+       ]
+   ```
+
+3. Restart `isulad`:
+
+   ```bash
+   $ sudo systemctl restart isulad
+   ```
+
+4. Define `RuntimeClass CRD` for example:
+
+   ```yaml
+   apiVersion: node.k8s.io/v1beta1
+   kind: RuntimeClass
+   metadata:
+     name: kata-runtime
+   handler: kata-runtime
+   ```
+
+5. Define pod spec `kata-pod.yaml` for example:
+
+   ```yaml
+   apiVersion: v1
+   kind: Pod
+   metadata:
+     name: kata-pod-example
+   spec:
+     runtimeClassName: kata-runtime
+     containers:
+     - name: kata-pod
+       image: busybox:latest
+       command: ["/bin/sh"]
+       args: ["-c", "sleep 1000"]
+     hostNetwork: true
+   ```
+
+6. Run pod:
+
+   ```bash
+   $ kubectl create -f kata-pod.yaml
+   $ kubectl get pod
+   NAME               READY   STATUS    RESTARTS   AGE
+   kata-pod-example   1/1     Running   4          2s
+   ```
+
+   
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