Add config dir note in documentation

[theredcat]

When using only directory-based config, the exporter stops with an error:

$ ./exporter_exporter -log.level trace -config.dirs /etc/exporter_exporter/ --web.listen-address="0.0.0.0:8599"
ERRO[0000] open expexp.yaml: no such file or directory

If you provide an empty file or a file with an empty modules section, it crashes

$ cat test.yml 
modules:
$ ./exporter_exporter -config.file ./test.yml -config.dirs /etc/exporter_exporter/ --web.listen-address="0.0.0.0:8599"
panic: assignment to entry in nil map

goroutine 1 [running]:
main.setup(0x0, 0x0, 0x0)
        /home/tristan/src/exporter_exporter/main.go:174 +0xdce
main.main()
        /home/tristan/src/exporter_exporter/main.go:323 +0xd
$ ./exporter_exporter -log.level trace -config.file /dev/null -config.dirs /etc/exporter_exporter/ --web.listen-address="0.0.0.0:8599"
panic: assignment to entry in nil map

goroutine 1 [running]:
main.setup(0x0, 0x0, 0x0)
        /home/tristan/src/exporter_exporter/main.go:174 +0xdce
main.main()
        /home/tristan/src/exporter_exporter/main.go:323 +0xde

The other option is to set the default value of the config file to an empty string instead of "expexp.yaml" but that will break default configuration https://github.com/QubitProducts/exporter_exporter/blob/master/main.go#L45