Add descriptions about systemd-resolved option

[egmc]

Since https://github.com/prometheus-community/systemd_exporter/pull/119 has been merged, add descriptions for this feature and new flag.

[egmc]

I tried to follow current manner to write all metrics and expected cardinality for each metrics. Though it could be a bit burden for future documentation, and these information(metrics type, cardinality) is not important as users can retrieve it from metrics itself.

So may be we should keep this level's documentation only for core systemd metrics, and less description about additional collector would be there(e.g. add more section like Additional Collectors and add one line systemd_resolved: per exporter metrics with prefix sytemd_resolved .

[egmc]

@SuperQ Hi, I think at least I need to add a description about new flag option for future release. Should I fix the conflict and keep metrics description like watchdog metrics for now? I'm going to fix it if needed.

[egmc]

@SuperQ Hi, sorry for bother you again. I have noticed your recent active work.. so could you release a new version after some updates finished? There seems to be some other people waiting for new release.

I'm not sure how to do that, though I'm also interested in maintaining some community based exporters like this, so please let me know if anything I can do for that.

Thanks.