function entropy( sigma ) {
	if ( isnan( sigma ) || sigma <= 0.0 ) {
		return NaN;
	}
	return 1.0 + ln( sigma / SQRT2 ) + ( 0.5 * GAMMA );
}
Returns the differential entropy of a Rayleigh distribution.

@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