Dual booting another OS

boot.loader.grub.extraEntries = ''
  menuentry "Windows 7" {
    chainloader (hd0,1)+1
  }
'';

References:

• https://askubuntu.com/questions/22629/add-windows-7-to-boot-menu
• https://old.reddit.com/r/NixOS/comments/31lx3i/windows_and_nixos_dual_boot/cq2tlbf