Update lukes_skew_code.R

So, I don't know how to add the documentation properly, but I had a go at writing it anyway. I don't know what sort of tests you mean, or what sort of crap should go in the custom mean function (note - I flagged where it should go once it's written with a comment. Normal mean() is there atm). I don't know if it's needed, but the function has a bunch of warnings written into it for bad inputs. I guess your test stuff handles that though?