Document how score works

[xamgore]

It should be documented at least.

• Negative or Zero YAKE score LIAAD/yake#17
• YAKE score(relevance ) LIAAD/yake#56
• is the score a linear value or an inverse natural logarithm? LIAAD/yake#69

[houston-d]

I have submitted a PR for the original YAKE implementation to prevent negative scores LIAAD/yake#96.

I have linked it here in case it relevant for this implementation