docs: migrate config v1 to v2

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

docs/PLUGINS.md

@@ -50,7 +50,9 @@ section for your given plugin `[proxy_plugins.myplugin]`. The `address` must
 refer to a local socket file which the containerd process has access to. The
 currently supported types are `snapshot` and `content`.
 
-```
+```toml
+version = 2
+
 [proxy_plugins]
   [proxy_plugins.customsnapshot]
     type = "snapshot"
@@ -236,7 +238,7 @@ Plugins are configured using the `[plugins]` section of containerd's config.
 Every plugin can have its own section using the pattern `[plugins."<plugin type>.<plugin id>"]`.
 
 example configuration
-```
+```toml
 version = 2
 
 [plugins]
@@ -245,4 +247,27 @@ version = 2
 ```
 
 To see full configuration example run `containerd config default`.
-If you want to get the configuration combined with your configuration, run `containerd config dump`.
+If you want to get the configuration combined with your configuration, run `containerd config dump`.
+
+##### Version header
+
+containerd has two configuration versions:
+- Version 2 (Recommended): Introduced in containerd 1.3.
+- Version 1 (Default): Introduced in containerd 1.0. Deprecated and will be removed in containerd 2.0.
+
+A configuration with Version 2 must have `version = 2` header, and must have
+fully qualified plugin IDs in the `[plugins]` section:
+```toml
+version = 2
+
+[plugins]
+  [plugins."io.containerd.monitor.v1.cgroups"]
+    no_prometheus = false
+```
+
+A configuration with Version 1 may not have `version` header, and does not need fully qualified plugin IDs.
+```toml
+[plugins]
+  [plugins.cgroups]
+    no_prometheus = false
+```