Add PurgeCSS comments to tailwind.css

I often end up having a single css file for Tailwind projects with the below contents. It would be nice if the `tailwind.css` file could include those comments by default. That way I can just import `"tailwind/tailwind.css"` in my code and not need the CSS file.

```css
/* purgecss start ignore */
@tailwind  base;
@tailwind  components;
/* purgecss end ignore */

@tailwind  utilities;
```