Document how to declare reactive variable type

Closes https://github.com/sveltejs/svelte-preprocess/issues/144

site/content/faq/500-what-about-typescript-support.md

@@ -2,4 +2,10 @@
 question: What about Typescript support?
 ---
 
-You need to install a [community supported preprocessor](https://github.com/sveltejs/integrations#preprocessors) such as [svelte-preprocess](https://github.com/kaisermann/svelte-preprocess). Work is ongoing to improve [IDE support](https://github.com/sveltejs/language-tools/issues/83). You can also run type checking from command line with [svelte-check](https://www.npmjs.com/package/svelte-check).
+You need to install a [community supported preprocessor](https://github.com/sveltejs/integrations#preprocessors) such as [svelte-preprocess](https://github.com/kaisermann/svelte-preprocess). Work is ongoing to improve [IDE support](https://github.com/sveltejs/language-tools/issues/83). You can also run type checking from the command line with [svelte-check](https://www.npmjs.com/package/svelte-check).
+
+To declare the type of a reactive variable in a Svelte template, you can use the following syntax:
+```
+let x: number;
+$: x = count + 1;
+```