Difficulty table recommendations for 14key

[telperion]

After a discussion in Hibiki_ext's DP-oriented discord server, I now have on hand a list of 14key tables and their targeted level ranges/coverage. I formatted it similarly to the wiki's Difficulty-Tables.md page. If you're interested in including it, feel free to just grab it and add 😄

https://github.com/telperion/beatoraja-english-guide/blob/add-dp-tables/Difficulty-Tables-14key.md

(I was going to just make a pull request but then I found out you can't do PRs to a github wiki lol)