chore: add base readme with basic contributing docs (#559)

Author: benmccann

README.md

@@ -0,0 +1,22 @@
+# svelte.dev
+
+This is the repository behind [svelte.dev](https://svelte.dev), the official Svelte site, and the related packages that it relies on.
+
+## Documentation PRs
+
+If you're creating a documentation PR, make sure you're targeting the right repository. More specifically, changes to content within `apps/svelte.dev/content/docs` are synced from other repositories, and documentation changes within those folder should therefore be made in those repositories:
+
+- `docs/svelte` -> https://github.com/sveltejs/svelte
+- `docs/kit` -> https://github.com/sveltejs/kit
+- `docs/cli` -> https://github.com/sveltejs/cli
+
+The tutorial, blog and examples are maintained within this repository.
+
+## Setup
+
+```
+pnpm install
+cd apps/svelte.dev
+USE_GIT=true pnpm sync-docs
+pnpm run dev
+```