Improve README to explain difference between how DockerHub images get created and Docker images for personal use

[karianna]

From @dinogun

The way it works currently is that, the scripts regenerate the dockerfiles each day, so they dont use the dockerflles in those dirs
Those dirs and the dockerfiles in them are only for any external folks who want to build their own images for whatever reasons
Also as a template for how to create docker images using the adopt tarballs
Since we generage the dockerfiles everyday based on the adopt API, the docker images and tarballs will always be in sync. Else the dockerfiles may be pointing to stale data