Add a quick install/update script

- I use this kind of thing quite a lot, grabs latest and bundles, but doesn't do a big all-of-gds style update
This won't actually update the local checkout. only the remote tracking branch.  You'll want to do a safe git pull here to ensure you don't blat any uncommitted changes etc. - see https://github.gds/gds/development/blob/master/single-update-git.sh for example.