Better documentation of "controller" mode

[regbo]

I just spent the better half of a day banging my head against the wall regarding the controller mode.

It looks like "controller" mode never fires up caddy, instead pushes changes to "servers". I understand that the docs say:

Controller
Controller monitors your Docker cluster, generates Caddy configuration and pushes to all servers it finds in your Docker cluster.

Standalone
This mode executes a controller and a server in the same instance and doesn't require additional configuration.

However, what I missed above is that a caddy server is never started in controller mode. I read it as:

Controller = caddy + control others Standalone = caddy + NOT control others

When really it is:

Controller = NOT caddy + control others Standalone = caddy + NOT control others

Perhaps some kind of small clarification would help? Perhaps:

Controller monitors your Docker cluster, generates Caddy configuration and pushes to all servers it finds in your Docker cluster. 

**Caddy proxy is never started and server nodes are required.**

[lucaslorentz]

That sounds good, can you please create a PR?