Adds font-feature-settings documentation

Forgot to open a PR for this part: basic documentation for #316. I don’t use Jekyll anymore so I didn’t actually manage to get this compiled. If someone else wouldn’t mind doing that, it’d really appreciate it. 

I can also add more examples if you think it’d be appropriate. On the CSS side of things `font-feature-settings`, has some pretty complex possibilities, but the mixin itself is simple.