Add detailed description of metrics (w.r.t summarization) and their applicable scenarios

[pfliu-nlp]

https://github.com/neulab/ExplainaBoard/blob/main/docs/supported_tasks.md#summarization

[noelchen90]

@pfliu-nlp Do you know where I can find this markdown https://github.com/neulab/ExplainaBoard/blob/main/docs/supported_tasks.md#summarization

[pfliu-nlp]

Hi, @noelchen90 After several rounds of refactoring, I think this file has been removed. This file owns a similar feature (a list of supported tasks). One idea to document metrics is to create a new markdown here: https://github.com/neulab/ExplainaBoard/tree/main/docs (supported_metrics.md for example) and describe them based on existing implementations: https://github.com/neulab/ExplainaBoard/tree/main/explainaboard/metrics

[pfliu-nlp]

I think maybe I would be a good fit to do this part, especially when detailing the differences between metrics. So, if you're happy with this, then I will take this issue.

[noelchen90]

@pfliu-nlp OK, cool, fine by me!