Add a simple mmctl README file (#23638)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/cmd/mmctl/README.md

@@ -0,0 +1,5 @@
+# mmctl
+
+`mmctl` is a remote CLI tool that lets you manage your Mattermost instance from the terminal.
+
+The project's documentation can be accessed within the [the Mattermost documentation](https://docs.mattermost.com/manage/mmctl-command-line-tool.html).