install database.yml template at startup
added postgresql support
README: document use of external postgresql server
Merge branch 'feature/postgresql-support'
added support for linking mysql and postgresql database containers
README: added section on linking mysql containers
README: added section on linking postgresql containers
Merge branch 'feature/database-linking'
README: complete the mysql and postgresql external database sections
README: added mysql server removal warning message
README: use the new docker command options.