docs: Explain how to add release notes

CONTRIBUTING.md

@@ -67,10 +67,30 @@ Check out the [security policy](https://github.com/NixOS/nix/security/policy).
    - [ ] API documentation in header files
    - [ ] Code and comments are self-explanatory
    - [ ] Commit message explains **why** the change was made
-   - [ ] New feature or incompatible change: updated [release notes](./doc/manual/src/release-notes/rl-next.md)
+   - [ ] New feature or incompatible change: [add a release note](#adding-a-release-note)
 
 7. If you need additional feedback or help to getting pull request into shape, ask other contributors using [@mentions](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#mentioning-people-and-teams).
 
+## Adding a release note
+
+Put a markdown file into `doc/manual/rl-next`. The name should roughly describe the feature so it's unlikely to collide with other PRs.
+The file should start with yaml frontmatter as specified below and can optionally contain text describing the change in detail.
+
+In general, make sure to format your release notes properly and link to relevant documentation whenever possible.
+Have a look at [the 2.20 release notes](https://nixos.org/manual/nix/unstable/release-notes/rl-2.20) for some examples.
+
+The yaml frontmatter can contain the following fields:
+
+- `synopsis`: (required) A short, one line description of the change.
+- `prs`: (optional) The numbers of the PRs that implemented the change. Can be just one.
+- `issues`: (optional) Issues that were closed as part of the change.
+
+_Be aware that `prs` and `issues` are space-separated lists of numbers, not yaml lists!_
+
+You can use [this template to get started](https://github.com/NixOS/nix/blob/b3b312827/doc/manual/rl-next/more-commands-respect-ctrl-c.md?plain=1).
+
+Refer to the [documentation for building the manual](https://nixos.org/manual/nix/unstable/contributing/documentation#building-the-manual)
+
 ## Making changes to the Nix manual
 
 The Nix reference manual is hosted on https://nixos.org/manual/nix.