Update README.md to note deduplication of events

[michael-danko-passport]

The existing event definition defines an event_id that is globally unique within the publisher's environment. This clarifies that

---

name: Default
about: Suggest changes to CDS
title:

---

CDS Pull Request

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Explain pull request

Please provide a clear and concise reason for this pull request and the impact of the change

Is this a breaking change

A breaking change would require consumers or implementors of the API to modify their code for it to continue to function (ex: renaming of a required field or the change in data type of an existing field). A non-breaking change would allow existing code to continue to function (ex: addition of an optional field or the creation of a new optional endpoint).

• Yes, breaking
• No, not breaking
• I'm not sure

Impacted Spec

Which API(s) will this pull request impact?

• Curbs
• Events
• Metrics

Additional context

Add any other context or screenshots about the feature request here.