Update README to include bundle install for dev kit itself

Merged username-removed-18385 requested to merge aaron465/gitlab-development-kit:readme into master

Currently the README doesn't mention the fact that you need to run bundle install on the dev kit to get foreman etc.

WRT markdown style guide - I've gone for the 80 char width option because I write python usually 👍 There already seems to be a mixture in this file, so I guess thats not too important so long as it renders correctly?