added migration step to readme

Author: SniperSister

README.md

@@ -5,5 +5,7 @@ This is the server for automated updates of Joomla CMS instances running on joom
 * Install docker and docker compose
 * Check out this repo
 * Start the webserver and DB services:  `docker-compose -f docker-compose.prod.yml up -d` 
+* Apply the database migrations: `docker-compose run --entrypoint="php artisan migrate" php`
 * Daemonize the queue worker using supervisord; the call in question is: `docker-compose run --entrypoint="php artisan horizon" php`
 * Add a cron job to execute the task scheduler. The scheduler should be trigger every 5min, the call is: `docker-compose run --entrypoint="php artisan schedule:run" php`
+