Document command-line completion installation

This is a draft based on [the same doc for docker-compose](https://github.com/docker/compose/blob/master/docs/completion.md). It still needs changes, but will get the thing moving.

Signed-off-by: David Stosik <david.stosik+git-noreply@gmail.com>

docs/completion.md

@@ -0,0 +1,62 @@
+<!--[metadata]>
++++
+title = "Command-line Completion"
+description = "Install Machine command-line completion"
+keywords = ["machine, docker, orchestration, cli,  reference"]
+[menu.main]
+parent="smn_workw_machine"
+weight=10
++++
+<![end-metadata]-->
+
+# Command-line Completion
+
+Docker Machine comes with [command completion](http://en.wikipedia.org/wiki/Command-line_completion)
+for the bash shell.
+
+## Installing Command Completion
+
+### Bash
+
+Make sure bash completion is installed. If you use a current Linux in a non-minimal installation, bash completion should be available.
+On a Mac, install with `brew install bash-completion`
+
+Place the completion scripts in `/etc/bash_completion.d/` (`` `brew --prefix`/etc/bash_completion.d/`` on a Mac), using e.g.
+
+    files=(docker-machine docker-machine-wrapper docker-machine-prompt)
+    for f in "${files[@]}"; do
+      curl -L https://raw.githubusercontent.com/docker/machine/v$(docker-machine --version | awk 'NR==1{print $(NF-1)}')/contrib/completion/bash/$f.bash > `brew --prefix`/etc/bash_completion.d/$f
+    done
+
+Completion will be available upon next login.
+
+<!--
+### Zsh
+
+**Note: there does not seem to be any Zsh completion file yet, but when there is, documentation could look like this.**
+
+Place the completion scripts in your `/path/to/zsh/completion`, using e.g. `~/.zsh/completion/`
+
+    mkdir -p ~/.zsh/completion
+    files=(docker-machine docker-machine-wrapper docker-machine-prompt)
+    for f in "${files[@]}"; do
+      curl -L https://raw.githubusercontent.com/docker/machine/v$(docker-machine --version | awk 'NR==1{print $(NF-1)}')/contrib/completion/zsh/$f > ~/.zsh/completion/_$f
+    done
+
+Include the directory in your `$fpath`, e.g. by adding in `~/.zshrc`
+
+    fpath=(~/.zsh/completion $fpath)
+
+Make sure `compinit` is loaded or do it by adding in `~/.zshrc`
+
+    autoload -Uz compinit && compinit -i
+
+Then reload your shell
+
+    exec $SHELL -l
+
+-->
+
+## Available completions
+
+**TODO**