Update docker completion (#5354)

* Update the docker completion script with the official docker zsh completion script
* Update README

plugins/docker/README.md

 ## Docker autocomplete plugin
 
-- Adds autocomplete options for all docker commands.
-- Will also show containerIDs and Image names where applicable
-
-####Shows help for all commands
-![General Help](http://i.imgur.com/tUBO9jh.png "Help for all commands")
-
-
-####Shows your downloaded images where applicable
-![Images](http://i.imgur.com/R8ZsWO1.png "Images")
-
-
-####Shows your running containers where applicable
-![Containers](http://i.imgur.com/WQtbheg.png "Containers")
-
-
-
-Maintainer : Ahmed Azaan ([@aeonazaan](https://twitter.com/aeonazaan))
+A copy of the completion script from the
+[docker](https://github.com/docker/docker/tree/master/contrib/completion/zsh)
+git repo.