add sync-docs README

Author: Rich-Harris

apps/svelte.dev/scripts/sync-docs/README.md

@@ -0,0 +1,82 @@
+# sync-docs
+
+Documentation lives in external repos (at the time of writing, [`sveltejs/svelte`](https://github.com/sveltejs/svelte), [`sveltejs/kit`](https://github.com/sveltejs/kit) and [`sveltejs/cli`](https://github.com/sveltejs/cli), though the plan is to add others) and synced into this repo.
+
+The repos must be cloned (or linked — see [next section](#setup)) into the `apps/svelte.dev/repos` directory.
+
+## Setup
+
+For local development, it's recommended that you symlink the repos, so that you can see changes immediately then open a PR once you're happy with them:
+
+```bash
+cd apps/svelte.dev
+ln -s /path/to/wherever/you/cloned/sveltejs/svelte repos/svelte
+ln -s /path/to/wherever/you/cloned/sveltejs/kit repos/kit
+ln -s /path/to/wherever/you/cloned/sveltejs/cli repos/cli
+```
+
+I have no idea what the equivalent Windows command would be.
+
+## Syncing
+
+To run the `sync-docs` script locally:
+
+```bash
+pnpm sync-docs
+```
+
+If you've symlinked the repos and want to watch for changes, you can use the `-w` command:
+
+```bash
+pnpm sync-docs -w # or --watch, if you're fancy
+```
+
+To pull from the remote first (or clone it, if you don't have it symlinked/cloned locally already):
+
+```bash
+pnpm sync-docs -p # or --pull
+```
+
+You can combine flags, and specify individual packages:
+
+```bash
+pnpm sync-docs -pw kit cli
+```
+
+## Automation
+
+Changes to documentation in the source repos will trigger the [`sync-docs.yml`](../../../../.github/workflows/sync-docs.yml) workflow. This is possible because the repos in question have their own `sync-request.yml` workflow that dispatches a request to this repo.
+
+To set up a new repo, create a file in that repo called `.github/workflows/sync-request.yml` with the following contents:
+
+```yml
+name: Dispatch sync request
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  dispatch:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Repository Dispatch
+        uses: peter-evans/repository-dispatch@v3
+        with:
+          token: ${{ secrets.SYNC_REQUEST_TOKEN }}
+          repository: sveltejs/svelte.dev
+          event-type: sync-request
+          client-payload: |-
+            {
+              "package": "cli"
+            }
+```
+
+You will need to add a `SYNC_REQUEST_TOKEN` to the repo. First, create a personal access token by going to https://github.com/settings/personal-access-tokens/new:
+
+- change the 'Resource owner' to `sveltejs`
+- provide access to the `sveltejs/svelte.dev` repository
+- under 'Repository permissions', change 'Contents' to 'Read and write'
+
+Then, go to `https://github.com/<ORG>/<REPO>/settings/secrets/actions`, click 'New repository secret' and paste in your newly generated personal access token.