add update instructions

CONTRIBUTING.md

@@ -35,6 +35,23 @@ Adapted from the [Contributor Covenant] and [The Carpentries Code of Conduct]:
 [Contributor Covenant]: https://github.com/EthicalSource/contributor_covenant/blob/cd7fcf684249786b7f7d47ba49c23a6bcb3233eb/content/version/2/1/code_of_conduct.md
 [The Carpentries Code of Conduct]: https://github.com/carpentries/docs.carpentries.org/blob/4691971d9f49544054410334140a4fd391a738da/topic_folders/policies/code-of-conduct.md
 
+## Updating reference manuals
+
+With the current setup, the Nix manual hosted on nix.dev does not get updated automatically with new releases.
+The following manual steps are required:
+
+- Update the inputs to use the latest versions of the Nix release branches with `nix flake lock`
+ - Make sure Nix can be fetched from the cache, otherwise it has to be rebuild.
+ If it doesn't, find the latest commit that is [built by Hydra](https://hydra.nixos.org/project/nix).
+- On each new Nix release, update the latest version in [`flake.nix`](./flake.nix) and [`source/reference/nix-manual.md`](./source/reference/nix-manual.md).
+- If Nixpkgs unstable or a NixOS stable release adopt a new version of Nix, update the corresponding references here.
+
+ Also update URLs to the the Nix manual to the version used by Nixpkgs unstable.
+ For example, if one wants to move from 2.18 to 2.19:
+ ```bash
+ sed -i 's#https://nix.dev/manual/nix/2.18/#https://nix.dev/manual/nix/2.19/#g' $(ls **/*.md)
+ ```
+
 ## What you can do
 
 ### You want to learn and use Nix?

flake.nix

@@ -3,6 +3,8 @@
 
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.05";
  inputs.flake-utils.url = "github:numtide/flake-utils";
+ # Various versions of the Nix manuals, see [](nix-manual)
+ # This requires human interaction to update, especially on new NixOS releases, see ./CONTRIBUTING.md
  inputs.nix_2-13.url = "github:NixOS/nix/2.13-maintenance";
  inputs.nix_2-17.url = "github:NixOS/nix/2.17-maintenance";
  # FIXME: the tip of the branch fails to build on Hydra, but we need it cached