Update README.md: add deprecation warning (#1551)

Author: linnan-github

README.md

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+**This technology is scheduled for deprecation.**
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+This repository will be archived and will no longer be updated.
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+See our [Update on Plans for Privacy Sandbox Technologies](https://privacysandbox.com/news/update-on-plans-for-privacy-sandbox-technologies/).
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+[Privacy Sandbox feature status](https://privacysandbox.google.com/overview/status) provides more information about the status of individual APIs and platform features.
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+---
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 # Attribution Reporting API
 
 _The Attribution Reporting API supports measurement of clicks and views with event-level and aggregate reports._