function skewness( r, p ) {
	return addon( r, p );
}
Returns the skewness of a negative binomial distribution.

@private
@param {PositiveNumber} r - number of failures until experiment is stopped
@param {Probability} p - success probability
@returns {NonNegativeNumber} skewness

@example
var v = skewness( 100, 0.2 );
// returns ~0.201

@example
var v = skewness( 20, 0.5 );
// returns ~0.474

@example
var v = skewness( 10.3, 0.8 );
// returns ~0.836

@example
var v = skewness( -2, 0.5 );
// returns NaN

@example
var v = skewness( 20, 1.1 );
// returns NaN

@example
var v = skewness( 20, NaN );
// returns NaN

@example
var v = skewness( NaN, 0.5 );
// returns NaN