Fix README instructtions to init a ConcreteDatastore project

[KhaledBousrih]

missing parts in README:

• Running a postgres Docker instance should specify the batabase name, the user name and the user password (the default values are in /development/settings.py in DATABASES):

docker run --name postgres-concrete -e POSTGRES_DB=db-concrete-datastore -e POSTGRES_USER=user-concrete-datastore -e POSTGRES_PASSWORD=pwd-concrete-datastore -d -p 5432:5432 postgres

• Before installing the package, make sure setuptools and pip are up-to-date:

pip install -U pip
pip install -U setuptools