Improvement: documentation for custom plugins

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?