Add ROUGE Metric Card
_The documentation is not available anymore as the PR was closed or merged._
Add ROUGE metric card.

I've left the 'Values from popular papers' section empty for the time being because I don't know the summarization literature very well and am therefore not sure which paper(s) to pull from (note that the original rouge paper does not seem to present specific values, just correlations with human judgements). Any suggestions on which paper(s) to pull from would be helpful! :) 