Document index.json mode for Svelte CSF

[JReinhold]

This PR updates the Readme to also mention that index.json mode is useful for testing stories not written as plain CSF stories.

This was prompted by #262.

[codecov]

Codecov Report

Base: 75.86% // Head: 75.86% // No change to project coverage 👍

Coverage data is based on head (5db7777) compared to base (942a3cc).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #263   +/-   ##
=======================================
  Coverage   75.86%   75.86%           
=======================================
  Files          11       11           
  Lines         174      174           
  Branches       36       36           
=======================================
  Hits          132      132           
  Misses         42       42           

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[jonniebigodes]

@JReinhold just needs a small tweak, and this should be good to go. Also, at some point in the future, we'll need to address the addon's documentation. But that's a whole different conversation that we can have later on.

Thanks for taking the time and effort to update the documentation. Appreciate it 🙏

[jonniebigodes]

All good. Thanks for taking care of the feedback so promptly. Appreciate it 🙏 ! Will merge this in a bit if you don't get around to it in the meantime @JReinhold

[github-actions]

🚀 PR was released in v0.10.0 🚀