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
The idea is simply to download the latest from each of those repos, definitely no desire to rebase etc. I didn't want to get into [this kind of chuff](https://github.gds/gds/development/blob/master/single-update-git.sh#L70-L84) for something so simple