Add readme note about global instance availability

[daun]

Describe the problem

• The global instance window.swup is not available immediately: #6
• Users might have to wait for the swup:enable event on the document before accessing it

Describe the proposed solution

• We should add a note about this and an example:

<script>
  console.log('swup not yet enabled', window.swup);
  document.addEventListener('swup:enable', () => {
    console.log('swup enabled', window.swup);
  });
</script>