add update instructions

CONTRIBUTING.md

@@ -22,6 +22,23 @@ 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 shell --run "niv update"`
+  - Make sure Nix can be fetched from the cache, otherwise it has to be rebuilt.
+    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 [`default.nix`](./default.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?

default.nix

@@ -27,6 +27,8 @@ let
     '';
     installPhase =
       let
+        # Various versions of the Nix manuals, grep for (nix-manual)=
+        # This requires human interaction to update, especially on new NixOS releases, see ./CONTRIBUTING.md
         inherit (inputs) nix_2-13 nix_2-17 nix_2-18;
         nix_2-13-doc = (import nix_2-13).default.doc;
         nix_2-17-doc = (import nix_2-17).default.doc;