Add info box regarding SELinux volume label

[Chris92de]

On host systems such as Fedora that make use of SELinux, it is requires to set a :z label for bind mounts so the host system allows Docker containers to interact with the contents of said folder.

This should be noted in the README under the How to use this image section.

Explanation in the Docker docs:
https://docs.docker.com/engine/storage/bind-mounts/#configure-the-selinux-label