Update documentation and code owners with the Nixpkgs core team

See <NixOS/org#216>.

Author: emilazy

.github/CODEOWNERS

@@ -1,2 +1,2 @@
 * @infinisil
-/members @NixOS/commit-bit-delegation
+/members @NixOS/nixpkgs-core

README.md

@@ -4,8 +4,8 @@ This repository publicly tracks the [current members](./members) and [changes](.
 of the [Nixpkgs Committers](https://github.com/orgs/nixos/teams/nixpkgs-committers) team,
 whose members have write access to [Nixpkgs](https://github.com/nixos/nixpkgs).
 
-The [Nixpkgs commit delegators](https://github.com/orgs/NixOS/teams/commit-bit-delegation)
-maintain the member list in this repository.
+The [Nixpkgs core team](https://github.com/NixOS/org/blob/main/doc/nixpkgs-core.md)
+maintain the member list in this repository according to the [documented procedures](https://github.com/NixOS/org/blob/main/doc/nixpkgs-committers.md).
 
 ## Nominations