function entropy( sigma ) {
	return addon( sigma );
}
Returns the differential entropy of a Rayleigh distribution.

@private
@param {PositiveNumber} sigma - scale parameter
@returns {number} entropy

@example
var v = entropy( 9.0 );
// returns ~3.139

@example
var v = entropy( 2.0 );
// returns ~1.635

@example
var v = entropy( -0.2 );
// returns NaN

@example
var v = entropy( NaN );
// returns NaN