chore(README): add no support message (#90)

* Update README.md

No support message

* Update README.md

Improve message prose

README.md

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+⚠️ ⚠️
+We are no longer mantaining this library. Feel free to fork it and work on your own branch.
+⚠️ ⚠️
+
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 # Segment plugin
 [![Pub Version](https://img.shields.io/pub/v/flutter_segment)](https://pub.dev/packages/flutter_segment)
 [![style: lint](https://img.shields.io/badge/style-lint-4BC0F5.svg)](https://pub.dev/packages/lint)