Removed "app/models" from "classmap" in composer.json

Since app/models directory has been removed, still having 'app/models' in the composer.json classmap  was causing:

[RuntimeException] Could not scan for classes inside "app/models" which does not appear to be a file nor a folder

Also, because I am an idiot and neglected to branch off my changes, this PR has a bonus of a README update to include the DB seeder step in the install.