Update README.md

Author: silesky

README.md

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-⚠️ Be sure to check out the next generation of analytics.js! https://github.com/segmentio/analytics-next 🎉 
+### ⚠️ This package is deprecated. Please migrate to: https://github.com/segmentio/analytics-next 🎉 
+###  End of life for security updates: August 31st, 2022
 If you have an existing JavaScript source with Segment, you can enable Analytics Next in the settings of the source.
 
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 # analytics.js-core
 
 [![CircleCI](https://circleci.com/gh/segmentio/analytics.js-core.svg?style=shield)](https://circleci.com/gh/segmentio/analytics.js-core)