docs: add release notes instructions for maintainers

Adds a brief instructions file to `.github/` documenting the prompt
and process for generating consistent release notes using an AI
assistant. Intended for use by maintainers when preparing releases.

Signed-off-by: Josh <josh.t.richards@gmail.com>

Author: joshtrichards

.github/release-notes-instructions.md

@@ -0,0 +1,32 @@
+# Release Notes Instructions
+
+When generating release notes for a new release, use the following prompt with GitHub Copilot (or another AI assistant), substituting the changelog and version number as appropriate.
+
+---
+
+## Prompt
+
+Generate release notes for **[version]** of the `nextcloud/documentation` repository based on the following changelog.
+
+For each PR in the changelog, look up the original (non-backport) PR to identify the true author. Organize the release notes into sections by which manual the PR touches: **Admin Manual**, **Developer Manual**, and **User Manual**. If a PR touches multiple manuals, include it in each relevant section with a summary appropriately tuned to that manual's audience. Eliminate any section for which there are no PRs.
+
+For each entry, write a concise human-readable summary (not just the PR title), and attribute it with `by @author in #number`.
+
+At the end, include a **New Contributors** section calling out any authors whose `author_association` on the original PR is `CONTRIBUTOR` (rather than `MEMBER`) — these are first-time or external contributors.
+
+All PRs are backported via `@backportbot` — include a note at the bottom acknowledging this and clarifying that original authors are credited throughout.
+
+**Changelog:**
+
+[paste changelog here]
+
+---
+
+## Notes
+
+- The changelog can be found on the [releases page](https://github.com/nextcloud/documentation/releases) or generated via GitHub's "Generate release notes" button as a starting point.
+- The key instructions that drive the correct output are:
+  1. **Look up original PRs** to get the real author, not `backportbot`
+  2. **Organize by manual**, not by type of change
+  3. **Cross-manual PRs appear in each relevant section** with tuned summaries
+  4. **`CONTRIBUTOR` association = new/external contributor** worth calling out