Add Sequence Diagram to README

[snaka]

Add a sequence diagram to help understand how Iron-session works.

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[snaka]

The position to add the diagram may not be appropriate, so please let me know if you have a request.

[vvo]

@snaka Would you be so kind to update the diagrams to only include app router ways of fetching data? And we would add the diagram at the end of the README, with a link at the top in the table of contents

[snaka]

@vvo

Sorry. I've moved away from the projects using this library, so it's been difficult to catch up with the latest Next.js behaviour.

If I can afford it, I will respond to request, but I can't promise that for sure.

If you want, someone can respond to this, or you can close this PR.

Thanks.

[vvo]

That's ok @snaka thanks a lot for the initial idea!