[docs] Exporter configuration syntax missing

[weakcamel]

First of all thanks for the exporter; I'm only just trying to set it up but so far it looks very very promising.

When in comes to configuration I had a bit of a hard time starting to be honest.

https://github.com/prometheus-community/ipmi_exporter/blob/master/docs/configuration.md seems to be possibly missing a section on what the configuration actully is

The exporter can read a configuration file by setting config.file (see above).

As it is, there's no 'above' - previous section speaks generally about local and remote endpoints, nothing about configuration. I ended up trying to configure the OS IPMI config file (/etc/freeipmi/freeipmi.conf) which worked but is of no consequence for the exporter. In the end I fortunately stumbled across the example configs and that helped.

It would be great to have a description of the config documented.