Format docker Readme like the other plugins.

plugins/docker/README.md

 ## Docker autocomplete plugin
 
+This plugin adds auto-completion for docker.
+
+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](https://github.com/docker/cli/blob/master/contrib/completion/zsh/_docker)
 git repo.