Document that stages w/o input are always considered out-of-date / are always rerun.

[RomeoV]

It was not clear to me from the docs that stages without an input stage are always rerun, thus invalidating any downstream stages.
I just spent half an hour debugging this, and haven't seen this documented clearly. Instead I found it here in the code:

dud/src/index/run.go

Lines 55 to 58 in 1b99eff

	if hasCommand && (len(stg.Inputs) == 0) {
	doRun = true
	runReason = "has command and no inputs"
	}

I think it would be good do document it in the dud run docstring, as proposed in this PR.

[kevin-hanselman]

Thanks for the contribution! I was hoping that the runReason display text would help with this. Were you unable to see it for some reason?

[RomeoV]

The run reason were not displayed, neither when adding "--trace" nor the "--verbose" flag. I agree they would have surely helped. However, i think it's still worth documenting.

[kevin-hanselman]

The run reason were not displayed

What version of dud are you using?

[RomeoV]

Indeed I was on an old version (0.4.3). With 0.4.5, I get the output!
Closing this PR then. Thanks!