cinelerra: add a note about parallel building

pkgs/applications/video/cinelerra/default.nix

@@ -50,6 +50,10 @@ stdenv.mkDerivation {
       fontconfig intltool
     ];
 
+  # Note: the build may fail with e.g.:
+  #   CXX      edl.o
+  # edl.C:50:25: fatal error: versioninfo.h: No such file or directory
+  #  #include "versioninfo.h"
   enableParallelBuilding = true;
 
   meta = {