Add F# 6 and NUnit example

[ggorlen]

https://deploy-preview-478--reverent-edison-2864ea.netlify.app/

[kazk]

You can add the page to https://github.com/codewars/docs/blob/e0e8a4bc8d40832cf7dfc19797ae11d7eb3e398d/sidebars.js#L364