Add cli reference docs for root-level commands

Signed-off-by: Vincent Demeester <vincent@sbr.pm>
(cherry picked from commit 9cea26bc77)
Signed-off-by: Victor Vieux <victorvieux@gmail.com>

docs/reference/commandline/stack.md

@@ -0,0 +1,39 @@
+---
+title: "stack"
+description: "The stack command description and usage"
+keywords: "stack"
+---
+
+<!-- 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
+     another repository, consider it read-only there, as it will
+     periodically be overwritten by the definitive file. Pull
+     requests which include edits to this file in other repositories
+     will be rejected.
+-->
+
+# stack
+
+```markdown
+Usage:  docker stack COMMAND
+
+Manage Docker stacks
+
+Options:
+      --help   Print usage
+
+Commands:
+  deploy      Deploy a new stack or update an existing stack
+  ls          List stacks
+  ps          List the tasks in the stack
+  rm          Remove the stack
+  services    List the services in the stack
+
+Run 'docker stack COMMAND --help' for more information on a command.
+```
+
+## Description
+
+Manage stacks.
+