Use Composer to manage sub-theme Bootstrap LESS dependencies when using LESS starterkit 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?
composer require twbs/bootstrap ^3.3.6 --prefer-dist -vvv

Drupal Bootstrap requires 3.x.x the caret will stop us tripping into v4.

We don't want to include any git files so we do --prefer-dist.

This downloads the latest bootstrap source to vendor/twbs.

I can expose this dir to my subtheme by editing:

/themes/custom/MYSUBTHEME/less/bootstrap.less

Now when I run composer update I will get the latest bootstrap less in my subtheme.