Mention possible challenges with UFW and firewalld

[antoineco]

A user reported issues running the stack due to firewalld blocking traffic on the elk bridge in #541. Another one reported similar problems in Gitter.

Similarly to what we did with SELinux, we should warn users about possible challenges using Compose on a machine running firewalld.

[antoineco]

Dropping this great article here for future references: https://www.howtogeek.com/devops/how-to-use-docker-with-a-ufw-firewall/