updated for composer dump-autoload

composer.json

{
    "name": "your name",
    "description":"a simple mini library demo app",
    "require": {
        "slim/slim": "^3.0"
    },
    "require-dev": {
        "phpunit/phpunit": "^7"
    },
    "autoload": {
        "psr-4": {
            "Library\\App\\" : "routes"
        }
    }
}