docs: announcement for ma in maintenance mode

packages/marketing-analytics-browser/README.md

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+# Announcement 📣
+
+The [Marketing Analytics Browser SDK]((https://www.docs.developers.amplitude.com/data/sdks/marketing-analytics-browser/)) is currently in maintenance mode. Amplitude has introduced the [Browser SDK 2.0](https://www.npmjs.com/package/@amplitude/analytics-browser) as its replacement. This SDK offers improved marketing attribution tracking, a simplified interface, and a more lightweight package.
+
+To learn more about the Browser 2.0 SDK, here are some useful links:
+
+* NPM: https://www.npmjs.com/package/@amplitude/analytics-browser
+* GitHub: https://github.com/amplitude/Amplitude-TypeScript
+* Documentation: https://www.docs.developers.amplitude.com/data/sdks/browser-2/
+
+Please check what has been improved and the detailed migration guide at the [here](https://www.docs.developers.amplitude.com/data/sdks/marketing-analytics-browser/migration).
+
 # @amplitude/marketing-analytics-browser
 
 Official Amplitude SDK for Web and Marketing Analytics