[ENH] clarify guiding principles for requirement levels

[sappelhoff]

closes #1422

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (8f21b45) 88.01% compared to head (a7397f5) 88.01%.

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Additional details and impacted files

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  Coverage   88.01%   88.01%           
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  Files          14       14           
  Lines        1268     1268           
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  Hits         1116     1116           
  Misses        152      152           

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[effigies]

Cross-posting from #1439:

[It] seems these could go in either the spec or the contributing guide. I'm slightly in favor of having spec-writing "principles" in the contributing guide as they have no effect on the interpretation of the spec or a dataset, but I would not attempt to block this.

[sappelhoff]

Fair point that it has no effect on the interpretation, but it does add some context and may make it easier for readers to think about the requirement levels. Having that said, I am also not absolutely convinced that it HAS to go in the spec. As long as we have it somewhere.

[Remi-Gau]

I have slight preference to have it in the spec if only because contributing if more contributors facing and less user facing.

I would say that this PR actually falls under the "interpretation" category.

[effigies]

I'm okay with this.