Documentation: mention that you will need a shim so tsc doesn't complain

If you're writing typescript Vue code, tsc will complain when you do `import Doc from 'Doc.md'`. You need to add a shim (ambient typescript module declaration) to tell tsc that a `.md` is a Vue component:
```
% cat src/shims-md.d.ts
declare module '*.md' {
import Vue from 'vue'
export default Vue
}
```

Otherwise when building, you will get errors about missing components.