Added an example of using the postgres base image

I spent quite a bit of time trying to figure out how to use the base image along with my own initialization scripts.  I thought that it may be worth putting in the README file for other people's benifit.
Wouldn't it be simple to just use the `POSTGRES_USERNAME` instead of `USERNAME`?
This way you can take advantage of what's already been done inside the Docker.

It would keep things simple to who is just checking out this image, since if would be more generic.

What do you think?