Adding bash completion and helper scripts

This patch adds some bash helper scripts.

 * docker-machine.bash - command completion for docker-machine
 * docker-machine-prompt.bash - function for putting the active machine
   name in PS1
 * docker-machine-wrapper.bash - function wrapper adding an `use`
   command that runs `eval $(docker-machine env whatever)` in the
   current shell.

Signed-off-by: David M. Lee <dlee@respoke.io>

docs/install-machine.md

@@ -57,6 +57,25 @@ to your PATH.
         $ docker-machine -v
         machine version 0.5.0 (3e06852)
 
+## Installing bash completion scripts
+
+The Machine repository supplies several `bash` scripts that add features such
+as:
+
+* command completion
+* a function that displays the active machine in your shell prompt
+* a function wrapper that adds a `docker-machine use` subcommand to switch the
+  active machine
+
+To install the scripts, copy or link them into your `/etc/bash_completion.d` or
+`/usr/local/etc/bash_completion.d` file. To enable the `docker-machine` shell
+prompt, add `$(__docker-machine-ps1)` to your `PS1` setting in `~/.bashrc`.
+
+    PS1='[\u@\h \W$(__docker-machine-ps1)]\$ '
+
+You can find additional documentation in the comments at the
+[top of each script](https://github.com/docker/machine/tree/master/contrib/completion/bash).
+
 ## Where to go next
 
 * [Docker Machine overview](/)