Summary:
The bash completion is simple but works both for ghc and ghci.
The README explains to the user what they have to do to get
it working (hopefully nothing).

Test Plan: Follow the README, then enjoy the cli completion in your terminal!

Reviewers: austin

Subscribers: thomie, carter, jstolarek

Differential Revision: https://phabricator.haskell.org/D536

GHC Trac Issues: #9005