Add to readme how to run with postgres

[peterdudfield]

It would be good to add to the readme how to run with a postgres url. This means testing with apps is more relastic

1. docker pull ghcr.io/openclimatefix/data-platform-pgdb:logging
2. docker run -it -e POSTGRES_PASSWORD=password -e POSTGRES_USER=postgres -p 5432:5432 -d postgres ghcr.io/openclimatefix/data-platform-pgdb:logging
3. export DATABASE_URL=postgresql://postgres:password@localhost:5432/postgres
4. make run

put we could make a relesae of postgres to make this cleaner

[devsjc]

There is now a postgres container being build, and the README points to both an example docker compose file and the configuration file where the environment variables are defined - I think that's enough guidance.