Add a reference for CRPS decomposition approach.

[seabbs]

          Add a reference for this approach. For example some mean CRPS decomposition discussed here; https://arxiv.org/pdf/2311.14122

_Originally posted by @seabbs in https://github.com/epiforecasts/scoringutils/pull/854#discussion_r1692024460_

[sbfnk]

In conversation with @jbracher he suggested to reference the WIS paper (which introduces the decomposition, if not for CRPS) for the approach implemented here. I agree that the question of where both of these sit with respect to the decompositions in the Arnold et al. paper is an interesting one.