select a software license or note terms in the README

[darrend]

First, thanks so much for putting this together! I used it to verify a custom IdP implementation in a client project.

Would you be willing to select and apply a License to this project? If you choose an open source route, my next step would be to capture my modest customizations in a private repo so we could easily test our IdP functionality in the future.

FWIW the change I made was to switch the sp image from image: leroymerlinbr/php:7.3-fpm-nginx to image: leroymerlinbr/php:7.4-fpm-nginx-v2. The latter supports the linux/arm64/v8 platform and runs on an Apple Silicon Mac. My private branch also adjusts default ports. It would be handy for me to lock in a dev sandbox app/solution using this code as the base.