Update documentation

[mholtzscher]

Update documentation to explain each readability score.

[ernesto-butto]

Hello!

First of all, thank you for creating this library.

I could help with this one, and also, I think it's important to add that it only works for English. I'm working on an app that requires readability, I´m using Flesch readability easy score, and calculating other languages (de, it, es, fr) using formulas specifics for those languages.

I wrongly assumed that this library worked for other languages, and realized only after I finished the code migration that it did not work for the other languages. So maybe we could update this ticket to

• Explain each readability score.
• Add a note for other languages besides English

What do you think?