bitbox: add comment explaining the need of Qt 5

Author: tensor5

pkgs/by-name/bi/bitbox/package.nix

@@ -11,6 +11,8 @@
 }:
 
 let
+  # Qt 6 doesn’t provide the rcc binary so we create an ad hoc package pulling
+  # it from Qt 5.
   rcc = runCommand "rcc" { } ''
     mkdir -p $out/bin
     cp ${lib.getExe' libsForQt5.qt5.qtbase.dev "rcc"} $out/bin