docs: add runtime format in config doc

docs/features/pouch_with_kata.md

@@ -64,6 +64,20 @@ You might need to modify this file, make sure that all binaries have right path
 
 With all the steps finish, you can play with kata container.
 
+1. add kata-runtime into config file, restart pouchd, ensure that pouchd know the specified runtime.
+
+```
+{
+    "add-runtime": {
+        "kata-runtime": {
+            "path": "/usr/local/bin/kata-runtime"
+        }
+    }
+}
+```
+
+2. start kata container.
+
 ```shell
 $ pouch run -d --runtime=kata-runtime 8ac48589692a top
 00d1f38250fc76b5e66e7fa05a41d342d1b48202d24e2dbf06b20a113b2a008c
@@ -73,7 +87,7 @@ Name     ID       Status         Created         Image
 00d1f3   00d1f3   Up 5 seconds   7 seconds ago   docker.io/library/busybox:latest   kata-runtime
 ```
 
-Enter into the kata container.
+3. enter into the kata container.
 
 ```shell
 $ pouch exec -it 00d1f3 sh

docs/features/pouch_with_runV.md

@@ -93,6 +93,18 @@ With runv related tools installed, we need to start pouchd. Then we can create h
 
 We can create hypervisor-based container by adding a flag `--runtime` in create command. And we can also use `pouch ps` to list containers including hypervisor-based containers whose runtime type is `runv` and runc-based containerd whose runtime type is `runc`.
 
+First, add runv into config file, restart pouchd, ensure that pouchd know the specified runtime.
+
+```
+{
+    "add-runtime": {
+        "runv": {
+            "path": "/usr/local/bin/runv"
+        }
+    }
+}
+```
+
 ``` shell
 $ pouch create --name hypervisor --runtime runv docker.io/library/busybox:latest
 container ID: 95c8d52154515e58ab267f3c33ef74ff84c901ad77ab18ee6428a1ffac12400d, name: hypervisor

docs/user/config_file.md

@@ -28,6 +28,23 @@ value of `--config-file`.
 * We allow users set slice or array type of flag simultaneously from command
   and config file line，and merge them.
 
+### Runtime format
+
+If user want to add more runtime into pouchd, add like:
+
+```
+{
+    "add-runtime": {
+        "runc": {
+            "path": "/usr/local/bin/runc",
+            "runtimeArgs": [
+                "--debug"
+            ]
+        }
+    }
+}
+```
+
 ### Steps to configure config file
 
 1. Install PouchContainer, you can find detail steps in [PouchContainer install](https://github.com/alibaba/pouch/blob/master/INSTALLATION.md).