Wrap nixpkgs with a warning

diff --git a/README.md b/README.md
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+# warn-nixpkgs
+
+Wrap nixpkgs, but emit a warning if this one is ever accessed.
+
+The Nix world is gradually adopting flakes and moving toward a world
+where there are no longer `nix-update`-maintained channels.  During that
+transition, it's nice to have system channels that still work, but emit
+a warning when they are accessed to alert the user of of a gap in their
+move-to-flakes.
+
+This repo can be used as a drop-in replacement for the `nixpkgs` channel.
+This repo doesn't contain `nixpkgs` -- this is merely a thin wrapper.
+Configure the actual `nixpkgs` to forward to as a `nixpkgs-warned`
+channel.

diff --git a/default.nix b/default.nix
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+args@{ ... }:
+let actualNixpkgs = import <nixpkgs-warned> args;
+in actualNixpkgs.lib.warn
+("Using legacy channel nixpkgs from " + <nixpkgs-warned>) actualNixpkgs