This doc has been moved to my blog: https://www.davidlanier.com/blog/2016/11/using-composer-on-pantheon
I worked through https://pantheon.io/blog/using-composer-relocated-document-root-pantheon and chased a few rabbits, and ultimately landed on what I think a pretty simple workflow. The main thing I was interested in was how to use composer to effectively manage a drupal project on pantheon. Composer is practically required for building the codebase of Drupal 8 because there are so many external dependencies now. Composer handles downloading the right versions of all packages.
The main steps:
- Create a new site at pantheon, selecting "Drops 8 Composer"