Copy “preview” documentation for “pre-release” publishing

docs/releasing/publishing-a-pre-release.md

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+# Publishing a preview of GOV.UK Frontend
+
+This preview guidance is aimed at Design System team members. If you're an external contributor who needs to create a preview, please [contact the Design System team](https://design-system.service.gov.uk/get-in-touch/) and we'll do it for you.
+
+Before you publish a preview, you need to have committed a code change to GOV.UK Frontend. Then follow these instructions.
+
+Use previews when you:
+
+- [work on developing a component or pattern](https://design-system.service.gov.uk/community/develop-a-component-or-pattern/) for the GOV.UK Design System
+- want to trial an experimental feature (guidance on trialing experimental features is in development)
+
+> **Warning** Your projects should never depend on a preview GOV.UK Frontend package. This is because someone could remove the GitHub branch containing the preview package at any time. For this reason, never use a preview package in a production setting.
+
+## What happens when you preview GOV.UK Frontend
+
+When you preview GOV.UK Frontend, this creates a GitHub branch. This branch contains the GOV.UK Frontend [`/packages/govuk-frontend`](/packages/govuk-frontend) directory with your trial changes.
+
+Projects can point to this branch in their package.json, instead of to the published [GOV.UK Frontend npm package](https://www.npmjs.com/package/govuk-frontend). No changes are published to the GOV.UK Frontend npm package as part of this process.
+
+## Publish a preview
+
+1. Run `git checkout -b BRANCH-NAME` to check out a new branch you want to preview, or `git checkout BRANCH-NAME` to check out an existing branch.
+
+2. Make any required changes and commit them.
+
+3. Run `nvm use` to make sure you’re using the right version of Node.js and npm.
+
+4. Run `npm ci` to make sure you have the exact dependencies installed.
+
+5. Run `npm run preview` to create and push a new branch that contains your changes. This process may take a few moments and will display a `Success!` message.
+
+## Preview your changes
+
+1. If you need to update an existing project to use the preview, copy the command that displays after the `Success!`message.
+
+2. Navigate to the project in the command line and run the success notification command. Running this command makes the project point to the preview branch, instead of to the published [GOV.UK Frontend npm package](https://www.npmjs.com/package/govuk-frontend). You can now preview your trial changes to GOV.UK Frontend.
+
+## Update a preview
+
+1. Check out the Git branch you previously previewed (this is the branch you work on, not the branch the script created).
+
+2. Make the required changes and commit them.
+
+3. Follow steps 3-5 in [Publish a preview](#publish-a-preview).
+
+4. Follow the steps in [Preview your changes](#preview-your-changes).