In Drupal 8 let's say I want to use a Bootstrap subtheme and the LESS starterkit described here: http://drupal-bootstrap.org/api/bootstrap/starterkits!less!README.md/group/subtheme_less/8
The Instruction there is to:
Download and extract the latest 3.x.x version of Bootstrap Framework Source Files into your new sub-theme. After it has been extracted, the folder should read ./subtheme/bootstrap.
Unfortunately this puts the Bootstrap source outside the ambit of my composer.json where I am managing my site dependecies and upgrade etc... more on why I want to do this here: https://bojanz.wordpress.com/2015/09/18/d8-composer-definitive-intro/
So how can I include the bootstrap source required by my subtheme in the composer.json?