Add typescript section

docs/migration-guide.md

@@ -4,7 +4,6 @@
 
 <!-- code_chunk_output -->
 
-- [Migration Guide](#migration-guide)
 - [From `v3` to `v4`](#from-v3-to-v4)
   - [Prepending content to `scss`](#prepending-content-to-scss)
   - [Executing some function before preprocessing](#executing-some-function-before-preprocessing)
@@ -88,4 +87,6 @@ sveltePreprocess({
 
 ### Type-checking components
 
-> TODO
+In `v3`, `svelte-preprocess` was able to type-check Svelte components. However, giving the specifics of the structure of a Svelte component and how the `script` and `markup` contents are related, type-checking was sub-optimal.
+
+In `v4`, your TypeScript code will only be transpiled into JavaScript, with no type-checking whatsoever. We're moving the responsibility of type-checking to tools better fit to handle it, such as [`svelte-check`](https://www.npmjs.com/package/svelte-check), for CLI and CI usage, and the [VS Code](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode) extension, for type-checking while developing.