Add DRONE_AGENT_PUBLISHED_PORTS to Documenation

[aebrat]

Spent the last few hours grappling with my agent docker containers deployed via autoscaler not having port 3000 exposed. As a result, drone could push jobs to the agent. I dove the source code and found the ENV var mentioned in the subject. It would have saved a lot of time if this was documented at autoscale.drone.io. Not sure how to go about contributing, figured it was worth a mention.

Furthermore, why does this var not have a default of 3000:3000 in the code? is this by design? or an overlooked issue? figure I may be doing something wrong and would like confirmation before opening a PR to make that change.