Add release docs

RELEASE.md

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+<!--
+SPDX-FileCopyrightText: © 2024 Matt Williams <matt.williams@bristol.ac.uk>
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+# Making a release
+
+Releases are completely automated by GitHub Actions:
+1. Make sure the [changelog](./CHANGELOG.md) is up to date.
+   A release cannot be made if there is nothing in the changelog in the `Unreleased` section.
+2. Go the the [Release](https://github.com/isambard-sc/conch/actions/workflows/release.yml) workflow page.
+3. Click "Run workflow" in the top right.
+4. Make sure the `master` branch is selected.
+5. In the box below, type `patch`, `minor` or `major`, depending on the [SemVer level](https://semver.org) of release to make.
+6. Press the "Run workflow" button.
+
+This will kick off a series of chained workflows, culminating in a new release appearing on the [Releases page](https://github.com/isambard-sc/conch/releases) and images and Helm charts added to the registry.