Basic explanation of systemd-style config support.

[pattivacek]

I just realized this was never updated. This is a pretty basic explanation; is there a definitive source that we should link to for systemd-style configuration?

[codecov-io]

Codecov Report

Merging #763 into master will not change coverage.
The diff coverage is n/a.

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[tkfu]

I like this, and would happily merge it as-is. I do have one or two clarifying questions, though:

1. What happens if I have a config file that doesn't specify a value for an option that gets processed after a config file that does? That is, does the overruling happen on the per-option level, or per-file level?
2. If I specify a config file on the command line, is that the only one that gets processed, or is it just that it's the last one that's processed?

[pattivacek]

@tkfu:

1. There are two steps. Searching by file name overrules per unique file name, then processing the options overrules per option.
2. Yes, if --config is specified at least once, the defaults are not used.

I made some meaning tweaks here: #766