* Readme: Improve docker-compose sample
- Add attributes "version" and "services" to make it a docker-compose.yml
- Add mariadb service with preconfigured user and database
Resolves https://github.com/gutmensch/docker-dmarc-report/issues/1
* Readme: Describe cron job schedule
It took me a while to figure out why nothing happened after starting the
container. This should help new users to understand that they only have
to wait for the cron job to start.
