Make sure you have git, composer, and terminus installed, then clone this repo, chmod +x d8_pantheon_init and cp d8_pantheon_init /usr/local/bin. Now you should be able to run the command d8_pantheon_init and it will interactively create a new pantheon d8 site for you in your current directory.
jacine/d8-pantheon-init
php script to automagically create a drupal 8 site on pantheon using the pantheon-systems/drops-8 repo
PHP