added reference (#23081)

src/renderers/shaders/ShaderLib/meshphysical.glsl.js

@@ -177,6 +177,8 @@ void main() {
 
 	#ifdef USE_SHEEN
 
+		// Sheen energy compensation approximation calculation can be found at the end of
+		// https://drive.google.com/file/d/1T0D1VSyR4AllqIJTQAraEIzjlb5h4FKH/view?usp=sharing
 		float sheenEnergyComp = 1.0 - 0.157 * max3( material.sheenColor );
 
 		outgoingLight = outgoingLight * sheenEnergyComp + sheenSpecular;