Add info about the bash completion script to the README

[bunnymatic]

Moved this issue over from the main project to the CLI

i'm not sure where exactly i'd put this but i wrote a
bash completion script for exercism CLI which would be
something that might be worth sharing?
https://gist.github.com/bunnymatic/8909929

@kytrinyx suggested I add this to the README which i will try to do soon.

[kytrinyx]

👍

[bunnymatic]

Instead I added it to the website - exercism/DEPRECATED.cli-www#13

Thanks for the merge @kytrinyx

[kytrinyx]

Yepp -- good call on putting it in the documentation that normal people will be reading!