Database credentials / Admin user credentials

[chougron]

I would add somewhere in the README that to be able to launch the database commands, you have to modify the .env files in tests/Application with your database credentials.

Also, I had to search in the database if there was any admin users created, and by chance the password is the same as the username, so I would also add in the readme that you can access the admin panel with the username sylius and the password sylius.

[pamil]

Could you open a PR with it?

[Prometee]

@chougron you have to use .env.local file instead to redefine your own env vars.