Clarify user-creation during installation

[rossjones]

The admin user was not created by following the documentation, and to get the admin user to appear, I had to run the db:seed command and provide the UserSeeder class.

This change clarifies that you should run that command.

[coreation]

Thanks @rossjones! But normally on "post composer install" (check composer.json) the db:seed command is called. This command adds, amongst other things, users to the database. However when an install is called with --no-scripts as an option, or the user forgot to fill in his database config, this won't work.

Could you perhaps add an extra sentence that states something "if no users were added during the install, you should call ....."?

Thanks!