Add explanation

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

@@ -11,8 +11,10 @@ let x: number;
 $: x = count + 1;
 ```
 
-To `import` a type or interface make sure to use [TypeScript's `type` modifier](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html#type-only-imports-and-export):
+To import a type or interface make sure to use [TypeScript's `type` modifier](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-8.html#type-only-imports-and-export):
 
 ```
 import type { SomeInterface } from './SomeFile';
 ```
+
+You must use the `type` modifier because `svelte-preprocess` doesn't know whether an import is a type or a value — it only transpiles one file at a time without knowledge of the other files and therefore can't safely erase imports which only contain types without this modifier present.