Add comment on interpretation of coef to PlackettLuce.Rd

[hturner]

Should add to PlackettLuce.Rd that mod$coefficients are the probabilities for each item that it is ranked first, given that first place is not tied. Then coef(mod) returns the log-probabilities, by default with the log-probability of the first item set to zero.

[hturner]

The model and the interpretation of its coefficients is now discussed in the vignette. This help file could probably still do with being updated though.