add readability formulas

[reynoldsnlp]

This is mostly already done.

Here are some resources: https://pdfs.semanticscholar.org/950c/b6af99ce6da269f012dd83980f686d5ca65b.pdf file:///~/Downloads/Assessment_of_Reading_Difficulty_Levels_in_Russian_Academ___.pdf https://link.springer.com/chapter/10.1007/978-3-030-04497-8_11 https://books.google.com/books?id=PUOCDwAAQBAJ&pg=PA141&lpg=PA141&dq=oborneva+russian+readability&source=bl&ots=BnnNYGzfbi&sig=ACfU3U0xnGHtsaleG5N2o39ArVnbOpESpQ&hl=en&sa=X&ved=2ahUKEwj76c-F74vnAhXIIjQIHR_9AiwQ6AEwAHoECAkQAQ#v=onepage&q=unav&f=false

[reynoldsnlp]

And webapps:

https://datayze.com/readability-analyzer.php http://www.analyzemywriting.com/