This adds documentation that was previously being drafted at
https://docs.google.com/document/d/1Pe3zkEZiYrtrjs7r9p0_bYFwnOO7bo0BlMYi27sakuk/edit?usp=sharing

Change-Id: I6ecd8e3e5a946960837fee8514ba130e01f9230c
Reviewed-on: https://chromium-review.googlesource.com/1023392Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>