docker: reformat README (#8286)

plugins/docker/README.md

-## Docker autocomplete plugin
+# Docker plugin
 
-A copy of the completion script from the
-[docker/cli](https://github.com/docker/cli/blob/master/contrib/completion/zsh/_docker)
-git repo.
+This plugin adds auto-completion for [docker](https://www.docker.com/).
+
+To use it add `docker` to the plugins array in your zshrc file.
+```zsh
+plugins=(... docker)
+```
+
+A copy of the completion script from the docker/cli git repo:
+https://github.com/docker/cli/blob/master/contrib/completion/zsh/_docker