Start on a migration guide (#500)

Author: benmccann

documentation/migrating/01-migrating.md

@@ -2,4 +2,15 @@
 title: Migrating
 ---
 
-SvelteKit is largely backwards-compatible with Sapper. However, there are a number of differences. See https://github.com/sveltejs/kit/issues/34 for more details until this page is fleshed out.
+SvelteKit is largely backwards-compatible with Sapper. However, there are a number of differences.YOu may find it helpful to view the [examples](https://github.com/sveltejs/kit/tree/master/examples) while migrating.
+
+- Remove `rollup.config.js` or `webpack.config.js` and replace with [`vite.config.js`](https://github.com/sveltejs/kit/blob/master/packages/create-svelte/template/vite.config.js)
+- Add a [`svelte.config.cjs` ](https://github.com/sveltejs/kit/blob/master/packages/create-svelte/template/svelte.config.cjs) with the adapter of your choice
+- Update your `template.html` to [`app.html`](https://github.com/sveltejs/kit/blob/master/packages/create-svelte/template/src/app.html)
+	- It should have a `<div id="svelte">` (or `id` matching `target` in `svelte.config.cjs`)
+- The error and layout pages should prefixed by `$` instead of `_`
+- Replace imports from `@sapper/app` with imports from `$app/navigation` and `$app/stores` and update APIs accordingly
+- If you have `src/node_modules/components`, move it to `src/components` and update the import path to `$components`
+- Move any custom `client.js` code to `$layout.svelte` and put it in an `if (browser)` check (`import { browser } from "$app/env";`).
+- `preload` has been renamed to `load`. It has a new method signature and return values
+- `sapper:prefetch` and `sapper:noscroll` have been renamed to `sveltekit:prefetch` and `sveltekit:noscroll`