Add cli reference docs for root-level commands

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

docs/reference/commandline/plugin.md

@@ -0,0 +1,44 @@
+---
+title: "plugin"
+description: "The plugin command description and usage"
+keywords: "plugin"
+---
+
+<!-- This file is maintained within the docker/docker Github
+     repository at https://github.com/docker/docker/. Make all
+     pull requests against that repo. If you see this file in
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+-->
+
+# plugin
+
+```markdown
+Usage:  docker plugin COMMAND
+
+Manage plugins
+
+Options:
+      --help   Print usage
+
+Commands:
+  create      Create a plugin from a rootfs and configuration. Plugin data directory must contain config.json and rootfs directory.
+  disable     Disable a plugin
+  enable      Enable a plugin
+  inspect     Display detailed information on one or more plugins
+  install     Install a plugin
+  ls          List plugins
+  push        Push a plugin to a registry
+  rm          Remove one or more plugins
+  set         Change settings for a plugin
+  upgrade     Upgrade an existing plugin
+
+Run 'docker plugin COMMAND --help' for more information on a command.
+
+```
+
+## Description
+
+Manage plugins.