Add explanation in comment

Former-commit-id: 5687281fdc6feccbfc4f141b6204226fd5758fe2

include/igl/writeOFF.cpp

@@ -61,6 +61,8 @@ IGL_INLINE bool igl::writeOFF(
 
   //Check if RGB values are in the range [0..1] or [0..255]
   int rgbScale = (C.maxCoeff() <= 1.0)?255:1;
+  // Use RGB_Array instead of RGB because of clash with mingw macro 
+  // (https://github.com/libigl/libigl/pull/679)
   Eigen::MatrixXd RGB_Array = rgbScale * C;
 
   s<< "COFF\n"<<V.rows()<<" "<<F.rows()<<" 0\n";