Improvement: documentation for custom plugins

[RecuencoJones]

I was just trying to build caddy binary with a custom set of plugins, but got stuck with the invalid import path "" error.

Looks like most of us are trying to use the qualified name of the plugin instead of just the last part of it (https://github.com/abiosoft/caddy-docker/issues/108#issuecomment-385743567). I.e:

RUN VERSION="1.0.3" \
    PLUGINS="http.prometheus" \
    ENABLE_TELEMETRY="false" \
    /bin/sh /usr/bin/builder.sh

Should actually be:

RUN VERSION="1.0.3" \
    PLUGINS="prometheus" \
    ENABLE_TELEMETRY="false" \
    /bin/sh /usr/bin/builder.sh

Can we add something to the documentation to avoid this?