Add source for luminance calculation

Author: nickygerritsen

webapp/src/Twig/TwigExtension.php

@@ -1165,6 +1165,7 @@ private function hexToForegroundAndBorder(string $rgb): array
         [$r, $g, $b] = $background;
 
         // Calculate relative luminance
+        // Source: https://www.w3.org/WAI/GL/wiki/Relative_luminance
         $r = ($r / 255 <= 0.03928) ? ($r / 255) / 12.92 : pow(($r / 255 + 0.055) / 1.055, 2.4);
         $g = ($g / 255 <= 0.03928) ? ($g / 255) / 12.92 : pow(($g / 255 + 0.055) / 1.055, 2.4);
         $b = ($b / 255 <= 0.03928) ? ($b / 255) / 12.92 : pow(($b / 255 + 0.055) / 1.055, 2.4);