normal_fragment.glsl: added explanation. See #8318.

src/renderers/shaders/ShaderChunk/normal_fragment.glsl

@@ -1,5 +1,7 @@
 #ifdef FLAT_SHADED
 
+	// Workaround for Adreno/Nexus5 not able able to do dFdx( vViewPosition ) ...
+
 	vec3 fdx = vec3( dFdx( vViewPosition.x ), dFdx( vViewPosition.y ), dFdx( vViewPosition.z ) );
 	vec3 fdy = vec3( dFdy( vViewPosition.x ), dFdy( vViewPosition.y ), dFdy( vViewPosition.z ) );
 	vec3 normal = normalize( cross( fdx, fdy ) );