BLEU metric card
_The documentation is not available anymore as the PR was closed or merged._ Some thoughts:
- For values, e.g. "Defaults to False", I would put False in code: `False`. Same for : "Defaults to `4`."
- I would put the following remark in "Limitations": 
> "BLEU's output is always a number between 0 and 1. This value indicates how similar the candidate text is to the reference texts, with values closer to 1 representing more similar texts. Few human translations will attain a score of 1, since this would indicate that the candidate is identical to one of the reference translations. For this reason, it is not necessary to attain a score of 1. Because there are more opportunities to match, adding additional reference translations will increase the BLEU score."

- Add some values from the original BLEU paper (https://aclanthology.org/P02-1040.pdf)
Add BLEU metric card