Add DOCKER_IP_ADDRESS env var description in README.md

[tiagostutz]

Hi!
I'm building a component that will run a NodeJS adapter along with a VerneMQ instance in the same container. I was struggling to make it run because the NodeJS service connects to tcp://localhost:1883 but the VerneMQ also needs to listen to external connections (por 1883 published).
Turns out that I needed to the VerneMQ process listen to 0.0.0.0 to make this internal/external conectivity work. Checking the code I could see that there's a DOCKER_IP_ADDRESS used as the IP_ADDRESS in the vernemq.conf file build process and them I just put DOCKER_IP_ADDRESS=0.0.0.0 and it worked.

Maybe there could be a mention at the README about this DOCKER_IP_ADDRESS environment var. If you agree I can send a PR on that.