top-level/linux-kernels.nix: add vendor kernels note

This codifies existing practice of avoiding adding new vendor kernels
to Nixpkgs.

Hopefully I've put this comment at the place in the file somebody
hoping to add a new vendor kernel would be most likely to look.

pkgs/top-level/linux-kernels.nix

@@ -57,6 +57,10 @@ in {
   kernels = recurseIntoAttrs (lib.makeExtensible (self: with self;
     let callPackage = newScope self; in {
 
+    # NOTE: PLEASE DO NOT ADD NEW VENDOR KERNELS TO NIXPKGS.
+    # New vendor kernels should go to nixos-hardware instead.
+    # e.g. https://github.com/NixOS/nixos-hardware/tree/master/microsoft/surface/kernel
+
     linux_mptcp_95 = callPackage ../os-specific/linux/kernel/linux-mptcp-95.nix {
       kernelPatches = linux_4_19.kernelPatches;
     };