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.
I'm not sure about this, but in my case after doing this it would create a $SCHEMA in the postgres (the default one) DB.
To avoid this I had do to it a little different, first do leave the schema creation out of the username/database creation.

Then I've done this:

This would make the postgres run in the new database, so when the schema is created it is done inside the new DB not the `posgres` one.
    gosu postgres postgres --single "$DB_NAME" <<- EOSQL
       CREATE SCHEMA "$DB_SCHEMA" AUTHORIZATION "$POSTGRES_USER";
    EOSQL