Add example with a placeholder (default slot) to docs

[matthew-ia]

thinking of something like a loading state that shows in place of the cayo before it loads.

[matthew-ia]

Is it worth supporting a prop or something that allows a component to self-render itself by default?

[matthew-ia]

Maybe add a section to the docs about "<Cayo> Slots", and how they're separate from your Cayo Component's (.cayo.svelte) slots, while explaining what you can do with them:

have something render within the by default, before the Cayo actually renders.

<Cayo src="Some.cayo.svelte">
  <div class="placeholder">
    <!-- placeholder content -->
  </div>
</Cayo>