Better documentation for running the functional tests

[darosior]

First, describe how to start a Postgre docker for the coordinator to quickly run the tests without setting up Postgre:

docker run --rm -d -p 5432:5432 --name postgres-coordinatord -e POSTGRES_PASSWORD=revault -e POSTGRES_USER=revault -e POSTGRES_DB=coordinator_db postgres:alpine
cargo build && POSTGRES_PASS=revault POSTGRES_USER=revault pytest -vvv -n 12 tests/

Maybe some other info are missing as well?