doc: mention adding maintainers must be done in a separate commit (#410292)

maintainers/README.md

@@ -40,6 +40,10 @@ In order to do so, add yourself to the
 [`maintainer-list.nix`](./maintainer-list.nix), and then to the desired
 package's `meta.maintainers` list, and send a PR with the changes.
 
+If you're adding yourself as a maintainer as part of another PR (in which
+you become a maintainer of a package, for example), make your change to
+`maintainer-list.nix` in a separate commit.
+
 ### How to lose maintainer status
 
 Maintainers who have become inactive on a given package can be removed. This

pkgs/README.md

@@ -113,6 +113,8 @@ Now that this is out of the way. To add a package to Nixpkgs:
 
    - Add yourself as the maintainer of the package.
 
+     - If this is your first time contributing (welcome!), [add yourself to the maintainers list](../maintainers/README.md#how-to-become-a-maintainer) in a separate commit.
+
    - All other [`meta`](https://nixos.org/manual/nixpkgs/stable/#chap-meta) attributes are optional, but it’s still a good idea to provide at least the `description`, `homepage` and [`license`](https://nixos.org/manual/nixpkgs/stable/#sec-meta-license).
 
    - The exact syntax and semantics of the Nix expression language, including the built-in functions, are [Nix language reference](https://nixos.org/manual/nix/stable/language/).