Command-line completion for Compose needs more instructions for Mac

[k33rs]

File: compose/completion.md

I've been installing for bash on Mac OS X. It works, but a change to the bash profile is also necessary, as reported in this post: http://davidalger.com/development/bash-completion-on-os-x-with-brew/

[drorata]

It worthy to also explain how to install the auto completion using MacPorts.

[jpooler]

#dibs for the hackathon

[mdlinville]

No action for two days, so this is up for grabs again!

[jpooler]

@mstanleyjones Sorry about the lack of movement on these, overcommitted myself a bit.

#2910 includes this issue.

[johndmulhausen]

Thanks for jumping back on it, @jpooler :)