chore(README): Lean Core badge

* docs: add Lean core badge

* docs: adjust badge style

README.md

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 [![All Contributors](https://img.shields.io/badge/all_contributors-16-orange.svg?style=flat-square)](#contributors)
 [![Known Vulnerabilities](https://snyk.io/test/github/react-native-community/react-native-webview/badge.svg?style=flat-square)](https://snyk.io/test/github/react-native-community/react-native-webview)
-<a href="https://www.npmjs.com/package/react-native-webview"><img src="https://img.shields.io/npm/v/react-native-webview.svg"></a>
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+[![Lean Core Extracted](https://img.shields.io/badge/Lean%20Core-Extracted-brightgreen.svg?style=flat-square)][lean-core-issue]
 
 **React Native WebView** is a modern, well-supported, and cross-platform WebView for React Native. It is intended to be a replacement for the built-in WebView (which will be [removed from core](https://github.com/react-native-community/discussions-and-proposals/pull/3)).
 
 This readme is available in:
 
 - [Brazilian portuguese](docs/README.portuguese.md)
+
+[lean-core-issue]: https://github.com/facebook/react-native/issues/23313