Document plan for handling vendor directory

[stevector]

@greg-1-anderson, with the composer vendor directory having been removed from Drupal 8.1.x, we should document how this repo will handle that change.

[greg-1-anderson]

Drupal 8.1.0 is live on Pantheon now. We commit the vendor directory back into Drops-8, because Pantheon requires that everything that is deployed to test and live must be in the repository.